18" SPR style gas length?

[beavis612]

I have searched the forum and not really found any real answers/opinions. I am building an 18" "SPR" type rifle. I am looking at the 18" BCM 410SS, and the 18" Rainier Ultramatch line. The part I am hung on is the gas length. The BCM is rifle length gas only, and the Rainier is rifle or intermediate length gas. From what I understand, the intermediate is 1.5" shorter than the rifle. What gas length is better suited to the 18" platforms? Is there any data out there to show the functional difference between the two? Thanks.

[agr1279]

Go with the rifle length. I spoke with people in the know and that is what I was told.

Dan

[Alex V]

When I built my "SPR" I used a Noveske barrel which also had an intermediate length gas system. Having never fired an AR with a rifle length system I can't attest to the difference in feel or reliability but I can say that I have had no issues with the intermediate length system.

It suckers every ammo I have ever given it from 55grain cheap stuff to 77gr Mk262, I usually feed it PPU 75gr because that is what my wallet likes the best. No functionality issues as said before.

From what I understand, traditionally it's supposed to be a rifle length system, but I would not put it past Noveske and Rainier to know what they are doing. :)

[beavis612]

Thanks for the replies. Does anybody know the reasoning behind the intermediate system? What issues was it designed to overcome?

[Rockhopper]

"Since the development of the 18" SPR barrel, there has not been a proper gas port placement to work with the desirable barrel length. Now there is a perfect combination, introducing the "Intermediate Length" gas system. Developed by Vltor Weapon Systems, and perfected by Noveske Rifleworks, the intermediate length gas system offers the ideal gas port placement, providing maximum accuracy and reliability. Overall, the intermediate length gas system (1.5" shorter then a rifle length) was developed from feedback from end users and industry professionals. Accuracy and reliability are the best characteristics of the intermediate length gas system."

[ldub5818]

How much longer is the intermediate opposed to a middy?

[beavis612]

1.9" longer than a middy AFIAK. The intermediate gas port should be 11.5" from the receiver face.

[mic2377]

I have an 18" Nordic rifle-gas barrel. It is very soft shooting, although a large part of this is the comp I have on the rifle. I have not had any functional issues, even with lots of different ammo including PMC, Lake City, and handloads, or under various conditions (-10F to 90F+). It also runs 100% with a C, H, or H2 buffer, or even an H3 if loads are hotter (ie 5.56 pressure). However, 3 gun folks tend to prefer a rifle gas barrel for its reduced recoil impulse.

Any of these barrels from a good maker will function without issue, I think you would be better served by focusing on the barrel profile and twist rate than by trying to over-analyze gas system length. I would recommend avoiding a heavy profile barrel.

[Techbrute]

The shorter gas system increases potential reliability by increasing dwell time. In the case lower pressure ammo, the shorter gas system can mean the difference between a complete cycle and a short stroke.

The longer gas system has the benefit of less perceived recoil and softer pulse.

I don't have any 18" barrels, but as an example, I have two rifles with 14.5" barrels, one with a carbine gas system, one with mid length gas. The mid length is more fun to shoot, but if I'm not shooting real 5.56mm, it will sometime short stroke.

So, if you like to go cheap on your ammo, definitely get the shorter one. I would choose the longer one.

[taliv]

I have had six 18" rifle gas systems and one KAC 18" (LPR?) which I'm not sure about. I was thinking it was rifle but might be intermediate. 4 are unfired but 2 of them i've shot for many years in competition. I'm a huge fan of rifle gas in 18" bbl. it's very soft and i've had no reliability issues.

I'm surprised at the "intermediate" comment from vltor above. Someone please correct me if I am wrong. I thought that "intermediate" was KAC's term that was basically just reusing the gas tube from their SR25s on their SR15s. Are they the same length as the vltor/noveske?

[MadAngler1]

I have searched the forum and not really found any real answers/opinions. I am building an 18" "SPR" type rifle. I am looking at the 18" BCM 410SS, and the 18" Rainier Ultramatch line. The part I am hung on is the gas length. The BCM is rifle length gas only, and the Rainier is rifle or intermediate length gas. From what I understand, the intermediate is 1.5" shorter than the rifle. What gas length is better suited to the 18" platforms? Is there any data out there to show the functional difference between the two? Thanks.

BCM is trying to duplicate the original Douglas 18" SPR barrels that had a 1/7 twist and a rifle length gas system (~12 inch). These uppers were slapped on M-16A1 lowers that had an A2 style buttstock (rifle length buffer tube, rifle buffer with a rifle buffer spring). Mind you, the original SPR set up was being used with an Ops Inc suppressor, without any sort of adjustable gas block (many used the PRI front sight base if I recall). Hence, running a suppressor on a rifle length gas system without any adjustability on already used M-16 lowers that have rifle length stocks is a hell of a lot more pleasant and reliable shooting experience than running one with a carbine length gas system and heavy "H" buffer. No special extra parts for the military to buy beyond assembling the uppers.

I believe Noveske's intermediate gas length system (~11 something inches) was meant to be used in conjunction with his adjustable "switch block" gas block, so one can run the gun suppressed and unsuppressed in conjunction with a carbine length stock. I'm sure it works fine with a non-adjustable gas block, given one uses the appropriately weighted buffer for appropriate cycling in whatever length buffer system the user chooses.

Recall that suppressing a rifle increases bolt/bolt carrier cycling speed due to the increased gas pressure from the use of the suppressor. In general, shorter gas length systems have a longer dwell time, shorter period of time for the extractor to pull the cartridge out of the chamber and faster cycling rate (assuming the buffer is the same), hence the increased stress on the M4's extractor. Rifle length gas systems are less prone to cycling issues than carbine length ones, hence the explosion in alternatively weighted heavy buffers for guys running carbine length gas systems and the increased popularity of mid length gas systems for guys running 16" carbines.

As far as perceived recoil, I doubt you'll feel the difference between a rifle length gas system and an intermediate or Noveske style one, assuming you are running the same buffer system. Both are heavy barreled options (BCM vs. Rainier ultramatch intermediate), and shouldn't budge much when shooting prone. Carbine vs. mid length or rifle is another issue.

I own a Noveske SPR 18" barrel with a switch block (which has their own proprietary gas system) in conjunction with a Larue 11" handguard, VLTOR MUR upper and a friggin A2 buttstock with a rifle length buffer and buffer tube/spring. :D It runs like butter and eats anything. Still waiting for the suppressor to try it in suppressed mode.

I don't think you can go wrong with either choice, just be aware of what stock/buffer system you're running with whatever barrel. Between the two, I would chose neither, because a 1/7 twist is far more appropriate for 77 grain SMKs than 1/8 twist in terms of stability at distance and terminal effects at distance. Centurion Arms sells a barrel like the original Douglas barrels, so I would check them out http://www.centurionarms.net/index.php?option=com_virtuemart&Itemid=139. I went with only because of the switch block option and because I may wind up putting a collapsible stock on my SPR. At least now VLTOR has the A5 buffer system which they claims runs as or more reliable than the rifle length M-16 one.

Note that ultimately, only the rifle length system as used in the military's spr is combat proven. To my knowledge the switch block and Noveske's is not (but I may be wrong). There is also no randomized control trial comparing the two systems either........

but god Bless John Noveske. May he R.I.P. :(

[beavis612]

Thanks for the replies. I am not trying to build a true MK12 or a clone. I will not be using a suppressor, or any of the correct OPS INC parts. This gun will use a Magpul UBR stock, and the barrel will be mated to a Vltor MUR upper. I think i am going to use the 18" Rainier Ultramatch with rifle length gas system.

[taliv]

Recall that suppressing a rifle reduces dwell time

it does? how?

[Dano5326]

SPR's, I have seen, mostly made with m16a1 lowers & stocks.

A rifle gas may give issue in extreme climatic conditions, elevation and cold. Limited dwell time. Certainly if you run cheap or dirty ammo. Port size, of course, key.

[MadAngler1]

Sorry for the f##@ ups. I fixed them in bold above. I had a few things backwards, I confused dwell time for the time the bolt has to extract its cartridge and that's what I get for writing this up in haste. I'm a f###@ing idiot and apologize.

Dwell time is defined as the time "high gas pressure" is expelled down the gas tube or gas piston. Dwell time increases in a shorter gas system compared to a longer gas system, when comparing across similar barrel lengths. Hence a carbine length gas system with a 16" barrel has more high pressure gas over a longer period of time up against the bolt carrier's gas key. Hence the higher cycling rate and shorter period of time the bolt has to extract the cartridge. Suppressors increase the speed the bolt carrier cycles even more as more gas gets sent down the gas tube before the bullet leaves the suppressor.

Thanks for the M-16A1 correction by the way. I thought about it after posting since I knew the lowers were mostly full auto and not burst only. Stupid me again.

[MadAngler1]

Thanks for the replies. I am not trying to build a true MK12 or a clone. I will not be using a suppressor, or any of the correct OPS INC parts. This gun will use a Magpul UBR stock, and the barrel will be mated to a Vltor MUR upper. I think i am going to use the 18" Rainier Ultramatch with rifle length gas system.

The UBR has a proprietary length buffer tube. I don't remember off hand what buffer is best depending on what gas system. I think a H or H2 should work fine with the rifle gas system, but you may have to ask around some more or consult Magpul or Slash: http://heavybuffers.com/reference.html. I'm sure the gun will run fine with the right buffer and spring.

[ldub5818]

I run my ubr with an h2, it is a middy gas piston system though. Runs like a sewing machine.

[justin_247]

I'm surprised at the "intermediate" comment from vltor above. Someone please correct me if I am wrong. I thought that "intermediate" was KAC's term that was basically just reusing the gas tube from their SR25s on their SR15s. Are they the same length as the vltor/noveske?

KAC's intermediate length is different than the one used by Noveske/VLTOR/Rainier.

Noveske's "intermediate" is .4" longer than KAC's "intermediate."

[MistWolf]

The UBR has a proprietary length buffer tube...

The UBR does not have a proprietary length receiver extension. It uses an entry receiver extension which is basically a fixed stock style extension shortened to use a carbine buffer and carbine spring.

Dwell time does not increase the amount of time the gas is in the gas tube. Dwell time as defined as the amount of time the bullet is in the barrel after passing the gas port has very little affect on BCG acceleration because the difference in time is a small fraction of a ,millisecond and the bullet has cleared the muzzle before the gases reach the expansion chamber of the BCG . The gases in the barrel are travelling at super sonic speeds but are slowed to the speed of sound when they pass through the gas port, slowing down how fast they can pressurize the expansion chamber.

The carbine length system does not have more time to pressurize the gas system if it's causing faster acceleration of the BCG because once the BCG opens a short distance, the gas tube disconnects from the gas key, cutting off flow to the BCG. As the BCG moves, the piston rings wipe past the vent ports of the carrier. Both events means the system begins venting. If the carrier is accelerated quicker, venting starts sooner and the gases have less time to pressurize the system.

What this means is the gas is carrying more energy. Since the bullet has cleared the muzzle before the gas system can be pressurized, it must be that the longer distance from port to muzzle increases the amount of time it takes the bore to vent back to ambient pressures.

This is why a suppressor increases BCG acceleration & speed. If it were simply the fact the bullet were in the bore longer, there would be no change in BCG acceleration when using a suppressor. In fact, dwell time, defined as the time the bullet is in the bore after passing the gas port, cannot have an affect on carrier acceleration at all. Because if the system were to be pressurized in that short time, the bullet would be in the exact position when it happened whether the muzzle were 4.5 inches or 45 inches from the gas port.

What actually happens is that the bullet clears the muzzle before the gases can pressurize the gas system and the bore begins venting. While venting happens rapidly, it's not instantaneous. It's the residual gas pressure that operates the rifle. With a short distance from muzzle to port, that section reaches ambient pressure sooner than it would with a greater distance.

Taken all together, what that means is increasing the time it takes the rifle to vent pressure by either increasing the port to muzzle distance or slowing the velocity of the gases through the use of a suppressor increases BCG acceleration & speed. The increase to BCG acceleration and speed is due to the gas pulse carrying more energy, not because it's given more time to act on the BCG.

Although it probably means it takes longer for the gas pulse to vent the expansion chamber as there is more gas to be vented. But the time it can act on the carrier will stop as soon as the bolt reaches full extension, if it hasn't finished venting by then

[Firefox]

From someone that currently runs an 18" SPR setup with a rifle gas system and VLTOR A5 Receiver extension I would highly recommend it. The recoil impulse is noticeably reduced compared to a carbine extension and is almost similar to using a rifle extension (I had an A2 stock on my rifle before switching to the A5).

As far as running reliably on different types of ammunition goes I cannot report any problems with any various brands of ammo I've used ranging from blackhills 77s to PMC 55s and american eagle 55s for plinking and when ammo has been hard to find lately. In fact, I cannot think of a single malfunction using the rifle although it's only seen a little bit over 1,000 rounds.

I realize that everyone has an opinion, but considering I had the same question when I was building my rifle I think you would be well served by the 18" rifle gas/A5 setup.

[koniz]

The intermediate length is nice, the hardest part about it is getting the gas tube and just talk to the ladies at Noveske to find that, the 18" intermediate Rainier barrel is nice.

[thehun]

I got a 18" WOA Match SPR 1:7 with rifle gas with H buffer

It ejects 5.56 3-3:30 o'clock

It ejects cheapo tula at 4:30-5 o'clock

[Alaska3006]

MK 12 uses a rifle length gas system........I put a Damaged Ind Gas Tube rifle length on my MK 12 mod 0

For buttstock the LMT SOPMOD and LMT Extension with the Damaged Ind H6 Buffer and Flat Wire CS buffer spring.