Question with bolt carrier lock position on new build

[bcs135]

So i just got to test fire my first AR build, everything seems to be working correctly, except for the BCG locking back completely.
On the last round it varies if it locks up completely with the bolt face behind the bolt catch or if it locks with the bolt face in front of the bolt catch. I'm assuming the bolt face should always lock up behind the bolt catch.
I'm running a 16" barrel with carbine length gas system with a sprinco blue spring and an H2 buffer. I was test firing using PMags and PMC 55gn rounds.
The gas systems looks good no fouling on the gas block so i don't think its a gas leak, would this be caused by my buffer? and if so is it cause by to heavy of a buffer so that it is short stroking or to light of a buffer an it is cycling fast enough that it cant' catch the full bolt quick enough.
Thanks in advance.

[Iraqgunz]

Greetings,

Please read this- https://www.m4carbine.net/showthread.php?t=85863 and then get back to us with a little more detail. Also, it would be really great if you would use capital letters and punctuation where appropriate. I know it's the internet but people will think that you are serious and not someone living in their grandmothers basement.

[bcs135]

Thought I had most of this in my post but to be explicit.

1. What make and manufacture is your AR? Was it pieced together (Frankengun)
Frankengun, this is my first build of an AR-15 type firearm. This is from the ground up, using info from this site to chose many of the parts I used in my build.

2. What modifications were made prior to actually shooting it? Did you put in a super high speed 3lb. buffer or a 100lb. buffer spring?
First time shooting, this is a virgin build.
16" 5.56 BCM light weight barrel.
VLTOR low profile gas block carbine length gas system.
Spring is the Sprinco BLUE spring
H2 buffer.

3. What type of ammunition did you use? Did you use reloads or absolute crappy cheap ammo made in Transylvania?
Tested fired using PMC Bronze 55gn 223 Rem FMJ.

4. Did the weapon function out of the box?
Virgin build this was my test run on the gun.

5. Were modifications made to the gas port?
No mods to gas port, using a VLTOR .625 diameter low profile gas block.


The BCG always locked back but this is what it looks like when the bolt does not fully lock.

[Tweak]

Drop one round in the chamber, press in on the bolt catch and fire the round holding the rifle as loosely as possible. Does the bolt lock back properly?

[VIP3R 237]

Personally i've had some problems with pmc when running a heavier buffer and spring. I wouldn't call pmc under powered but your setup (H2 carbine gs and blue springco) could probably run better with some full power 556 and get a little more omph in order to get a more consistent bcg lock.

[rjacobs]

PMC is known to be underpowered.

Any reason for the H2 buffer over an H?

What LPK is in the gun? I know there was talk about some weak bolt catch springs causing this issue.

To me though it sounds like weak ammo and to heavy of a buffer.

[Iraqgunz]

I believe you are correct. My feeling is that if he put the original spring back in or simply used an H buffer it would be fine.

The PMC stuff I have at home seems pretty good and when we ran it over the chrono it seemed like a decent load (but I don't have that info here).


PMC is known to be underpowered.

Any reason for the H2 buffer over an H?

What LPK is in the gun? I know there was talk about some weak bolt catch springs causing this issue.

To me though it sounds like weak ammo and to heavy of a buffer.

[bcs135]

Thanks for the input guys. LPK is from Rock River, I picked the H2 and BLUE spring for this build from one of the other threads here. It was the recommended setup for a carbine length gas system, probably running 5.56 thought.

I thought it was a buffer issue, just wasn't sure which way to go. One thread on here recommends heavier buffers for reliability, that may have been for feed issues. Another recommends to do the one round lock up test and if it doesn't lockup go lighter. I'll keep in mind the bolt catch spring thought too, did not think about that.

I'll pick up a H buffer and see how it cycles.

Thanks again.

[Iraqgunz]

I think you are misunderstanding or reading some things. We recommend heavier buffers and springs in AR's that are generally overgassed. BCM's don't fall into this category.

We also recommend them in many SBR's- again doesn't apply here.

I would start with the cheapest item first- replace the Springco spring with a standard GI spring. Then I would test fire it. If that doesn't work then I would look at the buffer.

You shouldn't have any issues selling that Springco spring to someone else not the H2 buffer.


Thanks for the input guys. LPK is from Rock River, I picked the H2 and BLUE spring for this build from one of the other threads here. It was the recommended setup for a carbine length gas system, probably running 5.56 thought.

I thought it was a buffer issue, just wasn't sure which way to go. One thread on here recommends heavier buffers for reliability, that may have been for feed issues. Another recommends to do the one round lock up test and if it doesn't lockup go lighter. I'll keep in mind the bolt catch spring thought too, did not think about that.

I'll pick up a H buffer and see how it cycles.

Thanks again.

[rjacobs]

if he put the original spring back in

I was under the impression that the blue spring is a standard rate, at least its sold as a standard rate.

I think if he switches to an H and uses some better ammo, even the federal bulk stuff from walmart, he will be good. I bought a large amount of PMC a few years ago for cheap(it was cheap at the time) and compared to XM193 it had WAY less felt recoil. I know that is a subjective evaluation of how much power ammo has, but it was a noticeable difference(it also stank like ammonia REALLY bad, like shitty commie ammo). I ran this ammo through my DDM4 with no issues though.

I personally always start with an H buffer and standard spring(like I said I dont think the Sprinco Blue spring is the issue since its supposed to be a standard rate spring, just higher quality). I also break in the gun with about 500 rounds of XM193(or my clone hand load) since it has some decent power and should assure that the gun runs well.

[markm]

Spring is the Sprinco BLUE spring


Who told you to run this?

[Iraqgunz]

It may be, I don't pay attention and I know that I used a Springco in my original SBR set up with an H3.

In any case I think that the H buffer would be more than adequate. I would still try the standard spring.


I was under the impression that the blue spring is a standard rate, at least its sold as a standard rate.

I think if he switches to an H and uses some better ammo, even the federal bulk stuff from walmart, he will be good. I bought a large amount of PMC a few years ago for cheap(it was cheap at the time) and compared to XM193 it had WAY less felt recoil. I know that is a subjective evaluation of how much power ammo has, but it was a noticeable difference(it also stank like ammonia REALLY bad, like shitty commie ammo). I ran this ammo through my DDM4 with no issues though.

I personally always start with an H buffer and standard spring(like I said I dont think the Sprinco Blue spring is the issue since its supposed to be a standard rate spring, just higher quality). I also break in the gun with about 500 rounds of XM193(or my clone hand load) since it has some decent power and should assure that the gun runs well.

[callen3615]

Ive never had a problem with pmc short stroking or anything else. I shoot TULAmmo and dont have any problems. Id look for a lighter buffer spring.

[bcs135]

Thought I would update this post, I did finally get a chance to test out my rifle again, not a lot of options in State College for ranges so it took a while to get out.
Anyways, I kept the blue Sprinco spring and swapped in an H buffer, works perfectly now locks back correctly every time.
Now I just need some range time to enjoy my new rifle. Thank you all for your input, as always this site is a wealth of great info.

[Iraqgunz]

So basically the H buffer and spring was fine. The H2 was too much and causing the problem. I assume that you tried different ammo as well?


Thought I would update this post, I did finally get a chance to test out my rifle again, not a lot of options in State College for ranges so it took a while to get out.
Anyways, I kept the blue Sprinco spring and swapped in an H buffer, works perfectly now locks back correctly every time.
Now I just need some range time to enjoy my new rifle. Thank you all for your input, as always this site is a wealth of great info.

[bcs135]

Yes the H2 was too much. Did not get a chance to try different ammo yet, I just got an order of Federal XM193, its going to be a while until I can get out to the range again though.