Anyone tried running a 20" 5.56 rifle upper with carbine lower?

My father put an M4 collapsible stock with a carbine buffer tube On his 20" Bushmaster and it runs fine.

Additionally, the Canadian military has the same setup with their C7A1 & A2 series of rifles.

Assuming a proper size gas port, the key factor is what's known as dwell time, or cork time. This is the interval between when the bullet passes the gas port and when the bullet leave the barrel. This is the time when the gas system is pressurized. The dwell time of a 20" barrel with a rifle-length gas system is approximately the same as the dwell time of a 14.5" barrel with a carbine-length gas system or a 16" barrel with a mid-length gas system. If you're going to use a carbine receiver extension, I'd recommend trying an H buffer and a standard carbine action spring.

However, if you had the option, the rifle-length receiver extension is preferable. There's also Vltor's A5 system which uses a receiver extension longer than a carbine extension but shorter than a rifle extension. The advantage is that you can use standard carbine collapsible stocks with the A5 system. It does use propietary buffers, but uses a standard rifle action spring.

I have three ARs set up with the A5 system; two are mated to 16" barrels with mid-length gas and one is mated to a 16" barrel with carbine-length gas. On the mid-length rifles, I use Vltor's H2 buffers. On the carbine length gun, I use their H3 buffer.

Ran one all last week, 20" Bushmaster and Colt 6920 lower. Working up 65gr Sierra loads, worked great!

I do this on a regular basis no issue. But in any case the worst is that you need a different buffer. The buffer is the tuning element of the ar recoil system and cheap. So if you have to change it not a big deal.

It's a nice setup. IMHO a 20" with collapsible stock, flat top and folding front sight with ACOG/LVPO and a FA fire control group with short throw and gas pedal would have been the ultimate evolution of the M16/M855 combo.

USMC worked with VLTOR to develop the A5 system and the Army had a kit in the system that used the standard carbine tube with a heavier buffer, they call it the H6 IIRC. I never saw either fielded as we transitioned to M4/M4A1s. It should be noted that the Mil solutions where built around full auto/3 round burst.

For you, I'd use an H buffer in a carbine extension if you already have that lower. BCM uses H buffers with their rifle length collapsing stock models and I've used an H buffer with a BCM M16A4 upper and 5.56 ammo.

The A5 set from Vltor (and the BCM version) is a highly versatile system and some smarter folks than I swear by it. I don't have one so I don't have an informed opinion on it.

yes. no issues.

Google Canadian C7a2. It is (was) the configuration of the Canadian Army.

great set-up IMO.

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Another note on the Vltor A5 system; as mentioned it was developed with the Marines. It was supposed to be the recoil system for when the Corps transitioned from the M16A4 to the M16A5 (hence "A5" buffer system), which ultimately never happened. So to take an M16A4 setup and replace the buffer with an A5 would be right in line with why it exists.

The purpose of the A5 was to bridge the gap in reliability between the rifle and carbine setups. The longer stroke vs a carbine setup, along with higher mass, makes it less sensitive to the gas system being mistuned (aggressive gas port sizing, loose BCG seals, etc). It also has some biasing springs in it that address bolt bounce and a few other things, which standard rifle/carbine buffers do not have.

I have an A5 setup on two guns now (starting a third); they are super soft shooters and have relative disregard as to whether or not there is a suppressor present. I wouldn't hesitate to recommend the A5.

That said - A5 components are relatively hard to get. Vltor ships a ton of stuff, but it all sells out relatively quick. BCM makes an intermediate extension that is A5 capable, Magpul's SR25 tube and UBR 2.0 stock are A5 compatible, I think V7 makes an A5 tube, SOLGW now makes their version of the A5 extension. For springs, as mentioned, a rifle spring works, Vltor I believe only endorses their spring and the Sprinco green rifle spring. Buffers are harder to get, the H0-H4 represent how many tungsten buffers are present like a normal buffer system. A5H2 is the recommended basis for starting, and you can use weights from places like HeavyBuffers (who also make their own A5 buffers, but without the Vltor secret sauce inside).

My 18" barrel rifle-length gas system AR15 originally came with a rifle buffer system and a A2 (?) stock. It cycled smoothly, but I didn't like the stock.

I changed the stock for a collapsible carbine-length buffer system stock. It cycled fine, but was a bit harsh. I went to an H buffer and it helped. When I started shooting suppressed, I went to an H2 buffer. But it still never felt as nice as my 20" rifle-length AR15 with a rifle buffer system.

So I changed the stock back to a rifle-length buffer. The 18" barrel was noticeably more enjoyable to shoot.

I recently replaced the 18" barrel's traditional rifle spring/weight system with JP's silent spring system, using an H3 weight. Woohoo, now this rifle cycles really nicely.

Buffer length, weight, and type make a difference in how an AR15 feels. I've experienced no differences in reliability, but the feel is quite different.

quote:

Originally posted by Rustpot:
For springs, as mentioned, a rifle spring works, Vltor I believe only endorses their spring and the Sprinco green rifle spring.

I don't know why they would do this, because the A5 system uses a rifle-length action spring. In all three of my A5 setups, I use Colt OEM rifle action springs.

quote:

A5H2 is the recommended basis for starting...

Yes, which is why I use Vltor's H2 buffers in my 16" mid-length guns. For the same reason, I recommend the carbine H buffer in 16" mid-length gas guns (as well as 14.5" carbine-length gas guns) which use a carbine-length receiver extension.

In my 16" carbine gas system rifle, I use Vltor's H3 buffer, since this configurtion is over-gassed. This is the reason I use an H2 carbine buffer (as opposed to an H buffer) in my 16" carbine gas guns which use a carbine receiver extension. IOW over-gassed systems get a heavier buffer, whereas properly-gassed systems get Vltor H2 or carbine H buffer, depending upon the type of receiver extension. A shooter can tweak as necessary from there, depending upon brass ejection patterns.

For my 16" carbine gas A5, I use a Magpul UBR gen2 stock, which is actually the correct length for the A5 system. This is why your UBR gen2 stock comes with a spacer in order to use a carbine action spring. In order to use the UBR gen 2 as an AR5 setup, all you need is a rifle action spring an a Vltor buffer, and leave out the Magpul spacer. This is the reason that I was chomping at the bit to get a gen2 UBR, and why I was so frustrated that they kept pushing back the intro date of the revised stock. I really wanted to try out this setup.

And yes, the A5 makes for a smooth-shooting rifle, especially in 16" mid-length gas.

As to all of the above, well, as the saying goes, your mileage may vary, but I've put a lot of thought into these various setups.

quote:

Originally posted by parabellum:

quote:

Originally posted by Rustpot:
For springs, as mentioned, a rifle spring works, Vltor I believe only endorses their spring and the Sprinco green rifle spring.

I don't know why they would do this, because the A5 system uses a rifle-length action spring. In all three of my A5 setups, I use Colt OEM rifle action springs.

I'm not sure why, either, But that came from Nick W. at Vltor. If I had to guess it's to avoid liability on recommending parts outside their control, with the exception being Sprinco. Alan D. at Sprinco has a pretty stellar reputation for quality, he's now up to two failed buffer springs out of however many thousand he's produced. Both of my running A5 setups are using the Sprinco Green, and I have Blue/Red/White in the rest of my carbine actions, and Sprinco extractor and ejector springs in most of my BCGs.

Aren't they pretty much traveling the same distance?

For anyone who wants a good explanation of how the A5 system splits the difference between rifle and carbine action lengths, scroll to the bottom of this page and read post #300.

Here's a great vid of the difference between the 6 and 7 position VLTOR stocks.

On one of my mid-length guns, I have the VLTOR RE-10/A5SR 6 position receiver extension and a standard Magpul CTR stock. The stock collapses so that the extension doesn't stick out the back, but by its nature, there is a bit less than an inch of space between the front of the CTR stock and the end plate. On the other, I have the 7 position VLTOR RE-A5 receiver extension and VLTOR's EMOD stock. I really like this stock.