Popularity of the 12.5" carbine-gas SBR?

[Rayrevolver]

40% is the increase from 10.5" to 11.5" per Paul from BCM. Not sure that same number applies 11.5 to 12.5.

I did the math a while back based on the a 20" rifle, M193. 10.5 to 11.5 is 33% increase. 11.5 to 12.5 is 24%. 10.5 to 12.5 is a 65% increase in dwell.

Baseline (20" Rifle aka 20R)
20.0" 0.184ms

Mid Length Gas
11.5” 0.053ms 71% less dwell time (or 29% of the dwell time of the baseline)
12.5” 0.079ms 57% less dwell time
14.5” 0.139ms 25% less dwell time
16.0” 0.181ms 2% less dwell time

Carbine Gas
10.5” 0.090ms 51% less dwell time
11.5” 0.120ms 35% less dwell time
12.5” 0.149ms 19% less dwell time
14.5” 0.206ms 12% MORE dwell time
16.0” 0.248ms 35% MORE dwell time

[juliet9]

They all work with the correct gas port size, some just better than others. The cyclic rate historically has been higher the shorter the barrel. Modern "mechanisms" like the A5 stock and such helps correct some of the deficiencies - to a point. Physics are physics and there is only so much that can be done.

Running a 10.3 hard is tough on the weapon, the toughest. When it isn't yours, but the taxpayer's rifle, no worries.

12.5" is a happy medium between 10.3" and a 16" mid, functions well, and suppresses well.

edited to add, so the increase in dwell time is linear? 10.5 to 12.5 would be 32%? Pretty substantial if so.

note the difference between 11.5 and 12.5, also substantial, but between 12.5 and 14.5, not so much.

[tom12.7]

I did the math a while back based on the a 20" rifle, M193. 10.5 to 11.5 is 33% increase. 11.5 to 12.5 is 24%. 10.5 to 12.5 is a 65% increase in dwell.

Baseline (20" Rifle aka 20R)
20.0" 0.184ms

Mid Length Gas
11.5” 0.053ms 71% less dwell time (or 29% of the dwell time of the baseline)
12.5” 0.079ms 57% less dwell time
14.5” 0.139ms 25% less dwell time
16.0” 0.181ms 2% less dwell time

Carbine Gas
10.5” 0.090ms 51% less dwell time
11.5” 0.120ms 35% less dwell time
12.5” 0.149ms 19% less dwell time
14.5” 0.206ms 12% MORE dwell time
16.0” 0.248ms 35% MORE dwell time

Just out of curiosity, what curve did you base this on? A reference may help.

[Clint]

Just out of curiosity, what curve did you base this on? A reference may help.

Most likely this

http://www.ar15barrels.com/prod/operation.shtml

[tom12.7]

Possibly so, I do like the 12.5" 5.56 carbine gas AR base for a few reasons.

[BufordTJustice]

Clint and Tom, what are your opinions on 12.5" or 13" mid length barrels as a concept, assuming gas port size and geometry would be optimized?

Sent from my SPH-L720T using Tapatalk

[tom12.7]

Around 13.5"ish is about my lower length limit for a mid length gas 5.56 AR for most. Depending on the selected ammunition, chamber, throat, bore, port, possible cans, action, etc.. You can go less than that if you can accept more compromises, but the uses become more specific and less general with compromises similar for the function of the system.

[Clint]

Clint and Tom, what are your opinions on 12.5" or 13" mid length barrels as a concept, assuming gas port size and geometry would be optimized?

Sent from my SPH-L720T using Tapatalk

Too short for general purpose use, but just about perfect for dedicated suppressed use.

Unsuppressed, there is enough gas drive available, but timing is less than ideal.

[Diamondback]

Almost 20 years ago I had some 12.5", perhaps the first USG ones, uppers made for M4's. The reason was simply as short as possible while able to mount the m203 (9" barrel) using the barrel knotch mount of that era. A side benefit was the increased length made for a better size stick to pumel non-compliants with.

We did not change the gas port size and had no issues.

OAL w/ suppressor is handier vs 14.5"

Also the MV is greatly improved over the 10.3"

There is certainly a potential reliability increase, considering the dwell time, with a wider range of ammunition and tempature ranges.

In short a happy medium

This a precursor to the GL-SSC?

I'd consider building one of these uppers, if the handguards weren't unicorns... granted, because NFA is a nuisance when you're surrounded by Leftofascist arsebandits I'd be mounting it on a pistol lower, but I'm kinda weird in that like Legos my "fun" is in the building stage.