View Full Version : Bolt inconsistently locking back on 10.3" carbine
I just got back from taking my Mk 18 pistol to the range for the first time. I was getting an occasional failure to lock back with my handloads. This occurred with a 10 Gen M3 PMAG, a 30 round Gen M3 PMAG, a 30 round D&H mag w/magpul follower, and did not occur on a DD 32 round magazine. I fired about 100 rounds through the pistol today, mostly using the 10 round magazine. None of the magazines were new, and all have functioned perfectly in other ARs. The failure to lock back only occurred about half the time, so it's possible it would have occurred with the DD magazine if I had fired more ammo with it.
Ejection seemed weak, but spent casings were ejecting a few feet away at 4:00.
AMMO:
LC 16 brass
CCI 41 primers
24.5 gr TAC
Hornady 55 gr SP
2.200 OAL.
GUN:
Daniel Defense 10.3" Mk 18 upper with 0.070" gas port (have not and will not disassemble the upper to verify; Brownell's and DD both list this spec)
Toolcraft NiB BCG with BCM enhanced extractor spring, insert, and o-ring
SIONICS H2 buffer
Springco Blue spring
My hypothesis is that the buffer spring is the culprit, and switching to a Springco White or White Hot spring will cure this issue. I figured I'd post here and see what you guys & gals think before I start buying springs.
Here is what I'd do:
Make sure everything is sealed up correctly - gas block, gas tube, carrier key. Remember - it isn't completely abnormal for there to be some soot around the gas tube/gas block juncture.
I generally test fire a new build in stock configuration. In this case, a carbine buffer and regular carbine spring. That way I'm not chasing stuff.
So I'd try with a carbine buffer, carbine spring first; if that works then I'd try an try an H with the regular carbine spring (or the H2 if you don't have an H). If that works then I'd diddle with the blue spring.
JM .02 - probably overvalued.
Bob Sacamano
07-07-18, 18:50
Would just clean, lube, then shoot some more. Will probably run like a champ once broken in a bit.
What are the Lower build specs?
What are the Lower build specs?
Centurion CM4 stripped lower
SIONICS Builder’s LPK (no trigger)
BCM mil spec trigger (not the PNT)
Generic end plate
Generic castle nut torqued to 39 ft lbs & staked
H2 buffer
Blue Springco spring
SBA3 brace w/included receiver extension
Before doing anything, get some factory 5.56 spec ammo and start with a lock back check.
Stock configuration for an AR does NOT include carbine weight buffers. Companies seeking to cut corners and save money use carbine weight buffers because they do not use tungsten weights. A properly gassed AR should run with an H2 buffer and Sprinco blue spring with 5.56 spec ammo.
Do you have another lower you know works? Do you have an upper you know that works you can test your pistol lower with?
FWIW - I am getting some instances of the bolt carrier getting "stuck" inside the buffer tube on my newly installed SBA3. It's likely due to tolerance stack but that is something else that would contribute to cycling issues.
Before doing anything, get some factory 5.56 spec ammo and start with a lock back check.
Stock configuration for an AR does NOT include carbine weight buffers. Companies seeking to cut corners and save money use carbine weight buffers because they do not use tungsten weights. A properly gassed AR should run with an H2 buffer and Sprinco blue spring with 5.56 spec ammo.
Do you have another lower you know works? Do you have an upper you know that works you can test your pistol lower with?
Granted, most rifles will work with the H or H2 buffer because many companies punch their ports larger to ensure the rifle works with a wide variety of ammunition. But I've been told the mil-spec is a .063 port and a carbine buffer. If I'm I working on faulty info could you point me to a source where I can plus up? Thanks.
If I did not plan on shooting 5.56 out of the rifle, I would just as soon make sure it functions with what I'm going to shoot out of it.
The firearm needs to function with what you feed it. In the OP's case apparently he handloads, if he has a load he is partial to, I'd try to get the rifle (pistol in this case) to run on what he is going to feed it.
If going to a carbine buffer solves the problem, do you think that putting spec 5.56 through it will lead to a situation where the bolt is outrunning the mag spring, etc.?
When Colt was developing the M4, they first tried the carbine buffer with 3 steel weights and it gave them fits. When carrier speed were sufficient to ensure reliable function, they got bolt bounce. To stop the bolt bounce, they had to increase buffer mass by replacing one steel weight with one made of tungsten. The M4 was originally designed to be a PDW to replace the M1 Carbine, not a primary infantry rifle. The minor flaws the early M4s had didn't show up until special forces units put the M4 hard use in combat. It was discovered that while the H buffer worked fine during semi auto use and occasional burst or full auto use, heavy useage showed that there still problems with bolt bounce, particularly with the heavier SOCOM barrel. Colt again increased buffer mass by using two tungsten and one steel weight giving us the H2 buffer. This information can be found in the book Black Rifle II by Chris Bartocchi.
I have verified for myself that the AR runs smoother with H and H2 buffers than the lighter carbine buffer. In fact, testing has me convinced that the carbine buffer is simply too light even when the AR is properly gassed. Using an upper with an SLR adjustable gas block, I tuned the upper until the bolt would eject and just lock back on an H buffer. I then tried a carbine buffer. Recoil was sharp as the buffer banged against the back of the RE. I closed the gas by one click. Now, the AR would eject, but not lock back. I opened the gas block one click and the AR once again ejected and locked back. I then tried it with the H2 buffer. The AR ejected and locked back using the same setting.
During this test, I noted that recoil with the carbine weight buffer was sharp. The buffer was bottoming out hard against the RE. Recoil was softer with the H buffer and softer still with the H2. I also tried the the A5 RE and an A5H2 buffer and Sprinco green spring. The A5H2 buffer is heavier than an H2 as it's the same weight as a rifle buffer. Recoil was softer still. As recoil is too sharp with the lightweight carbine buffer even with the gas adjusted to a minimum setting, I can come to no other conclusion that the carbine buffer is too light.
However, I felt that I had reached the point of diminishing returns with the H2 buffer with a carbine length spring and the A5H2 and rifle buffer using a rifle spring. If an AR is still over gassed with an H2, A5H2 or rifle buffer, it's time to address gas flow. Going any heavier will result in the reciprocating mass pushing the AR around excessively.
The first rule tuning an AR (or any machine), is to start from the base line to start from. In this case, the base line is an AR tuned to run on 5.56 spec ammo such as the Colt 6920. By starting off trying to get an AR to run on downloaded ammo, another variable is introduced, complicating troubleshooting. Once it's established that the AR will run on 5.56 with an H buffer and standard carbine spring, then the shooter can down load the ammo and make adjustments for reliable function.
A reliable AR is a balance of gas drive, reciprocating mass and spring rate. The gas drive is a balance of gas pressure, flow and volume. Changing the ammo changes all three. While the AR can be tuned to run on lower pressure ammo, doing so will give it a narrower span of operation, limiting the utility of the AR.
MWAG has a 10.5" barrel with a .070" gas port, an H2 buffer and Sprinco blue spring. That should work with 5.56 spec ammo. Once it's established whether or not the AR will run with baseline ammo, we can begin figuring out just where the problems lies.
Incorrect. The MILSPEC is a .063 gas port with an H buffer. New carbines are being shipped with the SOCOM barrel and use an H2 buffer with the same gas port.
Granted, most rifles will work with the H or H2 buffer because many companies punch their ports larger to ensure the rifle works with a wide variety of ammunition. But I've been told the mil-spec is a .063 port and a carbine buffer. If I'm I working on faulty info could you point me to a source where I can plus up? Thanks.
If I did not plan on shooting 5.56 out of the rifle, I would just as soon make sure it functions with what I'm going to shoot out of it.
The firearm needs to function with what you feed it. In the OP's case apparently he handloads, if he has a load he is partial to, I'd try to get the rifle (pistol in this case) to run on what he is going to feed it.
If going to a carbine buffer solves the problem, do you think that putting spec 5.56 through it will lead to a situation where the bolt is outrunning the mag spring, etc.?
I would use a standard MILSPEC BCG and not an NiB one and see what happens.
I just got back from taking my Mk 18 pistol to the range for the first time. I was getting an occasional failure to lock back with my handloads. This occurred with a 10 Gen M3 PMAG, a 30 round Gen M3 PMAG, a 30 round D&H mag w/magpul follower, and did not occur on a DD 32 round magazine. I fired about 100 rounds through the pistol today, mostly using the 10 round magazine. None of the magazines were new, and all have functioned perfectly in other ARs. The failure to lock back only occurred about half the time, so it's possible it would have occurred with the DD magazine if I had fired more ammo with it.
Ejection seemed weak, but spent casings were ejecting a few feet away at 4:00.
AMMO:
LC 16 brass
CCI 41 primers
24.5 gr TAC
Hornady 55 gr SP
2.200 OAL.
GUN:
Daniel Defense 10.3" Mk 18 upper with 0.070" gas port (have not and will not disassemble the upper to verify; Brownell's and DD both list this spec)
Toolcraft NiB BCG with BCM enhanced extractor spring, insert, and o-ring
SIONICS H2 buffer
Springco Blue spring
My hypothesis is that the buffer spring is the culprit, and switching to a Springco White or White Hot spring will cure this issue. I figured I'd post here and see what you guys & gals think before I start buying springs.
I just got back from taking my Mk 18 pistol to the range for the first time. I was getting an occasional failure to lock back with my handloads. This occurred with a 10 Gen M3 PMAG, a 30 round Gen M3 PMAG, a 30 round D&H mag w/magpul follower, and did not occur on a DD 32 round magazine. I fired about 100 rounds through the pistol today, mostly using the 10 round magazine. None of the magazines were new, and all have functioned perfectly in other ARs. The failure to lock back only occurred about half the time, so it's possible it would have occurred with the DD magazine if I had fired more ammo with it.
Ejection seemed weak, but spent casings were ejecting a few feet away at 4:00.
AMMO:
LC 16 brass
CCI 41 primers
24.5 gr TAC
Hornady 55 gr SP
2.200 OAL.
That's nearly a starting load, isn't it?
With Ramshot Tac and 55gr projectiles, I don't ever go lower than 25gr.
Try bumping up the charge and see what happens.
titsonritz
07-08-18, 05:08
Run some 5.56 ammo through it and see if the issue persists, if not there you go.
Before doing anything, get some factory 5.56 spec ammo and start with a lock back check.
Stock configuration for an AR does NOT include carbine weight buffers. Companies seeking to cut corners and save money use carbine weight buffers because they do not use tungsten weights. A properly gassed AR should run with an H2 buffer and Sprinco blue spring with 5.56 spec ammo.
Do you have another lower you know works? Do you have an upper you know that works you can test your pistol lower with?
The pistol lower I built is the only one I can legally use with the mk18 upper. I have a BCM upper and my dad has a DD upper, both 16" middies, that have been 100% reliable. The BCM is on a BCM lower with carbine spring and H1, and the DD is on a PSA lower with an unmarked buffer and their spring, presumably both carbine parts.
That's nearly a starting load, isn't it?
With Ramshot Tac and 55gr projectiles, I don't ever go lower than 25gr.
Try bumping up the charge and see what happens.
It's about midrange for .223, but I just checked Ramshot's website and it is indeed a starting load for 5.56. I can go as high as 27.3
http://www.ramshot.com/wp-content/uploads/2010/09/WesternLoadGuide1-2016_Web-1.pdf
If I did not plan on shooting 5.56 out of the rifle, I would just as soon make sure it functions with what I'm going to shoot out of it.
The firearm needs to function with what you feed it. In the OP's case apparently he handloads, if he has a load he is partial to, I'd try to get the rifle (pistol in this case) to run on what he is going to feed it.
Not partial to this load necessarily, but I am partial to handloading. I reload for around $0.12/round. Fortunately, it wouldn't add any significant cost to bump the charge weight up a grain or two.
Run some 5.56 ammo through it and see if the issue persists, if not there you go.
I test fired it with a round of Federal XM193 and it locked back with the DD magazine, but I realize that's too small a sample to give any relevant data on the ammo or magazine (though I suspect you guys are correct that a 5.56 load would lock back). I can try some more 5.56, but I gotta buy it first. I honestly only have the 5.56 on hand for a course in September. Otherwise, I handload for barely more than half the cost of steel cased ammo.
I can't afford to shoot a ton of factory 5.56, so I want this gun to function reliably with my handloads. 24.5 is convenient because I load on an XL650 and I have a 62 gr BTHP load that is sub MOA at this charge weight. It's a PITA to adjust the powder measure, but I'm more than willing to bump up the charge weight on the 55 gr load if that makes this gun happy.
I would use a standard MILSPEC BCG and not an NiB one and see what happens.
I'm genuinely curious why you say that. The toolcraft was merely a spare I had lying around until this build happened, but I would expect a BCG with a lower friction coefficient to lock back before a phosphate coated one.
Not being argumentative, just curious.
glockshooter
07-08-18, 10:32
I can't afford to shoot a ton of factory 5.56, so I want this gun to function reliably with my handloads. 24.5 is convenient because I load on an XL650 and I have a 62 gr BTHP load that is sub MOA at this charge weight. It's a PITA to adjust the powder measure, but I'm more than willing to bump up the charge weight on the 55 gr load if that makes this gun happy.
This may be the basis of your problem. If you are at the starting with Powder charge for .223 that it truly designed to be shot in a bolt gun. I certainly under the convenience side of what you’re saying but it sounds like your convenience is part of your problem. So in my opinion you have two choices. Either adjust your powder measure back and forth as needed or buy another tool head and powder measure you can switch. I’d be willing to bet you are causing your own problem. You could verify this but trying factory loaded ammo.
This may be the basis of your problem. If you are at the starting with Powder charge for .223 that it truly designed to be shot in a bolt gun. I certainly under the convenience side of what you’re saying but it sounds like your convenience is part of your problem. So in my opinion you have two choices. Either adjust your powder measure back and forth as needed or buy another tool head and powder measure you can switch. I’d be willing to bet you are causing your own problem. You could verify this but trying factory loaded ammo.
Just to clarify, this is a midrange .223 load, but a minimum 5.56 load. For .223 Ramshot lists 23.2 — 25.8, and for 5.56 they list 24.5 — 27.3 for 55 gr Hornady bullets seated at the crimp groove (2.200"). But you're right my current charge weight is likely the culprit in this gun.
http://www.ramshot.com/wp-content/uploads/2010/09/WesternLoadGuide1-2016_Web-1.pdf
I'm definitely going to try two things.
First, I'm going to buy a box of 5.56 and see if it locks a variety of my magazines back. Hypothesis: it will work perfectly
Second, I'm going to work up my handloads at .5 gr intervals up to 27.0 (max for 5.56 is 27.3). Hypothesis: the problem will probably go away at 25.0 and almost certainly go away at any charge higher than that.
I just got back from the range. Tested the gun with 5 different magazines (DD 32 round, Lancer L5AWM 30 round, Gen 3 PMAG 30 round, Gen 3 PMAG 20 round, and D&H 30 round). Loading one round into each magazine my dad and I shot 10 rounds of each ammo type.
Federal American Eagle 5.56 locked the bolt back 10/10 times
25.0 gr TAC locked the bolt back 4/10 times
25.5 gr TAC locked the bolt back 10/10 times
26.0 gr TAC locked the bolt back 10/10 times
26.5 gr TAC locked the bolt back 10/10 times
27.0 gr TAC locked the bolt back 10/10 times
Going forward I'll find a load between 26.0 and 27.0 that the 16" middies shoot accurately. That should give me plenty of wiggle room to find a decently accurate load that will still work when it's below freezing.
Great - sounds like your load was simply under gassed :)
Incorrect. The MILSPEC is a .063 gas port with an H buffer. New carbines are being shipped with the SOCOM barrel and use an H2 buffer with the same gas port.
Not that I'm questioning what you say, buy could you tell me where I can find that info - I'm a copy and paste kind of guy. Thanks.
I load on an XL650 and I have a 62 gr BTHP load that is sub MOA at this charge weight. It's a PITA to adjust the powder measure, but I'm more than willing to bump up the charge weight on the 55 gr load if that makes this gun happy.
I load on a 550 - same powder measure - I bought one of these and found it helpful in keeping track of things - https://dawsonprecision.com/arredondo-powder-measure-knob-for-dillon-precision-powder-measure/
Probably costs a nickel to make but $4.95 isn't much, I just threw it onto a sight order.
Anyways, tomorrow would you be will to PM me your recipe? 'Preciate it.
Not that I'm questioning what you say, buy could you tell me where I can find that info - I'm a copy and paste kind of guy. Thanks.
Colt uses a gas port of .0625" and an H buffer for the 16" 6920.
Colt uses a gas port of .0625" and an H2 buffer for the 14.5" M4.
I load on a 550 - same powder measure - I bought one of these and found it helpful in keeping track of things - https://dawsonprecision.com/arredondo-powder-measure-knob-for-dillon-precision-powder-measure/
Probably costs a nickel to make but $4.95 isn't much, I just threw it onto a sight order.
Anyways, tomorrow would you be will to PM me your recipe? 'Preciate it.
I've always wondered about the powder measure upgrades. I got so tired of adjusting the measure in the wrong direction that I wrote +/- arrows in sharpie, but it's still very much a guess & check operation. And I'm so anal that I weigh 10 charges electronically and won't stop adjusting until I get 250.0 gr with 10 throws for a 25.0 gr load.
Btw, your inbox is full.
I've always wondered about the powder measure upgrades. I got so tired of adjusting the measure in the wrong direction that I wrote +/- arrows in sharpie, but it's still very much a guess & check operation. And I'm so anal that I weigh 10 charges electronically and won't stop adjusting until I get 250.0 gr with 10 throws for a 25.0 gr load.
Btw, your inbox is full.
I think just having a dial with markings on it helps. I check mine just about the same way, and recheck the drop every second primer tube.
In terms of repeatability, this one would seem to be pretty accurate and repeatable: http://www.uniquetek.com/product/T1231 It is $129.00 installed on a new bar.
This is probably the next best thing: https://www.doublealpha.biz/click-adjustable-powder-knob
NiB is garbage. In addition NiB changes the dimensions of the components.
I'm genuinely curious why you say that. The toolcraft was merely a spare I had lying around until this build happened, but I would expect a BCG with a lower friction coefficient to lock back before a phosphate coated one.
Not being argumentative, just curious.
It comes from first hand examination of the carbines as well as measuring ports in barrels.
Not that I'm questioning what you say, buy could you tell me where I can find that info - I'm a copy and paste kind of guy. Thanks.
Mechanical devices fail, and sometimes mechanical devices are sold that dont meet spec.
Is this a gun that would take a A5 buffer system? If so, why not go there, it fixes so many ills, just why not.
Sorry if it's in middle of thread I skipped past.
PB
Crossed the 500 round mark yesterday and thought I’d provide an update.
Yesterday was my first time shooting this pistol in the cold (approx 28 degrees). I’m glad I did because it gave me some useful data. I fired 50 rounds of my 5.56 55 gr reloads and 50 rounds of my .223 62 BTHP reloads. Up until yesterday the .223 load had been flawless in the MK18, but after a few hours in the cold I started to have failures to lock back. The 55 gr 5.56 loads continued to lock back, but ejection was very weak (about 5 feet at 3:00). What is interesting is that the 62 gr load was fine at first, but started acting up after a few hours. I found it kind of fascinating to see how the cold affected function.
Going forward I will swap out the blue sprinco for a standard carbine power spring. I know Blue + H2 is often recommended, but I think the newer DD 10.3” barrels with the 0.070” ports are undergassed for that combo. I believe DD includes a standard spring and a regular H buffer with their complete MK18.
That .223 62 gr load is very accurate in our 16” rifles, and I have 750 rounds left, so I’ll address the overpowered spring and see how that goes. I know some will say it’s a 5.56 gun so feed it 5.56 ammo, but honestly I want absolute reliability with any ammo in any temp. Besides, the fact that the 5.56 rounds juuuuuust barely ejected has me thinking the gun is oversprung.
MIL MK18's use a USGI spring and H2 buffer.
Crossed the 500 round mark yesterday and thought I’d provide an update.
Yesterday was my first time shooting this pistol in the cold (approx 28 degrees). I’m glad I did because it gave me some useful data. I fired 50 rounds of my 5.56 55 gr reloads and 50 rounds of my .223 62 BTHP reloads. Up until yesterday the .223 load had been flawless in the MK18, but after a few hours in the cold I started to have failures to lock back. The 55 gr 5.56 loads continued to lock back, but ejection was very weak (about 5 feet at 3:00). What is interesting is that the 62 gr load was fine at first, but started acting up after a few hours. I found it kind of fascinating to see how the cold affected function.
Going forward I will swap out the blue sprinco for a standard carbine power spring. I know Blue + H2 is often recommended, but I think the newer DD 10.3” barrels with the 0.070” ports are undergassed for that combo. I believe DD includes a standard spring and a regular H buffer with their complete MK18.
That .223 62 gr load is very accurate in our 16” rifles, and I have 750 rounds left, so I’ll address the overpowered spring and see how that goes. I know some will say it’s a 5.56 gun so feed it 5.56 ammo, but honestly I want absolute reliability with any ammo in any temp. Besides, the fact that the 5.56 rounds juuuuuust barely ejected has me thinking the gun is oversprung.
MIL MK18's use a USGI spring and H2 buffer.
And if I remember correctly, they also have the same gas port size that MWAG says his barrel has. So that spring and buffer are probably the best bet.
MIL MK18's use a USGI spring and H2 buffer.
Yeah that’s what I’ll go with for now. If that doesn’t work I have an H1 to try.
As much as I want to do sprinco green and A5, I plan to put a Law Tactical folder on this, and I’m not sure I’ll like a LOP that long.
thegreyman
03-04-19, 07:23
I think this is a Reloading and Ammunition thread; thread began with function problems resulting from reloads not from factory ammunition.
I think this is a Reloading and Ammunition thread; thread began with function problems resulting from reloads not from factory ammunition.
I see where you’re coming from, but I wasn’t seeking reloading advice as much as I was seeking advice about my build. The ammo, while not necessarily full power 5.56, was not a problem in 3 other ARs. Although I could likely solve the issue by adjusting my reloads (or by simply purchasing factory 5.56 ammo), I’m more interested in advice about how to set up the gun optimally for my needs.
Yep that's the one.
And if I remember correctly, they also have the same gas port size that MWAG says his barrel has. So that spring and buffer are probably the best bet.
Remember when everyone was complaining about .080 ports on DD and Noveske barrels? I bet someone would trade barrels with you on EE. I don’t think you are going to optimize the current weapon to your ammo; it should work the other way around (I thought that was the point of handloading?).
10.3” barrels have a pretty narrow range of ammo they will tolerate because of having so little barrel after the port. Port size sets ammo type, and the Mil Mk18 was not built with light ammo in mind. I would bet if you put a regular spring and H2 in it, it’ll eat M855 without issue. If you must shoot lighter ammo, you might find a 10.x” with a slightly larger gas port more suitable. If you plan on shooting a range of ammo types, you might find that an 11.5” barrel is more forgiving. There’s a reason that length is popular on M4c, and among LE, whose issued ammo can be subject to change. Soldiers, Marines, Sailors, etc, don’t have to worry about that. Ammo only seems to get hotter with succesive generations.
10.3” barrels like to be suppressed...
I see where you’re coming from, but I wasn’t seeking reloading advice as much as I was seeking advice about my build. The ammo, while not necessarily full power 5.56, was not a problem in 3 other ARs. Although I could likely solve the issue by adjusting my reloads (or by simply purchasing factory 5.56 ammo), I’m more interested in advice about how to set up the gun optimally for my needs.
Remember when everyone was complaining about .080 ports on DD and Noveske barrels? I bet someone would trade barrels with you on EE. I don’t think you are going to optimize the current weapon to your ammo; it should work the other way around (I thought that was the point of handloading?).
10.3” barrels have a pretty narrow range of ammo they will tolerate because of having so little barrel after the port. Port size sets ammo type, and the Mil Mk18 was not built with light ammo in mind. I would bet if you put a regular spring and H2 in it, it’ll eat M855 without issue. If you must shoot lighter ammo, you might find a 10.x” with a slightly larger gas port more suitable. If you plan on shooting a range of ammo types, you might find that an 11.5” barrel is more forgiving. There’s a reason that length is popular on M4c, and among LE, whose issued ammo can be subject to change. Soldiers, Marines, Sailors, etc, don’t have to worry about that. Ammo only seems to get hotter with succesive generations.
10.3” barrels like to be suppressed...
I’m sure one of the overgassed barrels would work, but I’ll bump up the powder charge long before I do a barrel swap. It’s been reliable with the .223 stuff at the indoor range for about 200 rounds, but in the cold it was barely ejecting 5.56, which lead me to believe the extra power spring isn’t ideal.
thegreyman
03-07-19, 07:02
You want to set your MK 18 for light reloads?
Powered by vBulletin® Version 4.2.5 Copyright © 2024 vBulletin Solutions, Inc. All rights reserved.