Adj gas block adventures (1168)

[1168]

I shot some today with an adjustable gas block, and compared settings unsuppressed and suppressed with different springs and doodads.

I used an upper assembled with an A1 flash suppressor used to mount Griffin cans (MSDk, 30SDk,) a BRT 16” pencil barrel with longer-than-mid gas, and a Sentry 6 Ti click adjustable gas block. I used a FCD/Sionics BCG that has been used enough to trust. In two configurations, I used a LMT Enhanced BCG, recently rebuilt (springs, rings)

I got together 3 lowers; one with a Carbine RE, one with an A5, and one with a Rifle RE and A1 stock.

The following buffer weights were used:
Carbine buffer: 5.5oz
A5 buffer: 5.4oz
Rifle buffer:5.7oz
The rifle and carbine buffer were solid for consistency. The A5 was as it comes out of the box, with bias spring.

All numbers are minimum number of clicks open required to consistently achieve LRBHO with a Gen 3 PMag, and the rifle held firmly to my shoulder. This gas block has 3 clicks per turn, and a click is roughly 0.01” of adjustment screw travel. The first click is numbered 1, and so forth. Temp was ~60 degrees F.

Ammo used for testing was Federal XM855. I fired fouling shots with .223 handloads. I wiped off the bolt and carrier frequently to maintain consistency. No oil was used, and the gun started out dry.

[1168]

Rifle lower

Colt spring: 7
Colt Spring, M4SDk: 5
Colt spring, 30SDk: 5 (very close to not locking back)

36 coil Tubb spring: 6
36 coil Tubb spring, 30SDk: 5

42 coil Tubb spring: 7
42 coil Tubb spring, 30SDk: 6

[1168]

A5 lower

Colt spring: 7
Colt spring, 30SDk: 6

36 coil Tubb spring: 6
36 coil Tubb spring, 30SDk: 5

36 coil Tubb spring, LMT EBCG: 7
36 Tubb spring, LMT EBCG, 30SDk: 6

42 coil Tubb spring: 7
42 coil Tubb spring, 30SDk: 6

[1168]

Carbine lower

Colt spring: 7 (barely locked back)
Colt spring, 30SDk: 6

Sprinco Blue spring: 7
Blue spring, 30SDk: 6

36 coil Tubb spring: 7
36 coil Tubb spring, 30SDk: 6

42 coil Tubb spring: 7
42 coil Tubb spring, 30SDk: 6

[Disciple]

"Colt spring" is a rifle spring in the rifle and A5, and a carbine spring in the carbine?

From "7 (barely locked back)" and "42 coil Tubb spring: 7" I read that if anything the Colt spring provided more resistance. Manually cycling does it feel less smooth than other springs?

[georgeib]

I would never have guessed that a rifle lower would require less gas than a carbine. Interesting stuff. Thank you for posting that!

[1168]

"Colt spring" is a rifle spring in the rifle and A5, and a carbine spring in the carbine?

From "7 (barely locked back)" and "42 coil Tubb spring: 7" I read that if anything the Colt spring provided more resistance. Manually cycling does it feel less smooth than other springs?
Yes, rifle spring in the rifle and A5, and a carbine spring in the carbine. There is some variation from spring to spring with Colt springs, so it could be a fluke that it barely locked back a couple times. FWIW, I selected the longest carbine spring and longest rifle spring from my small collection of Colt springs. That may equate to newest, or stiffest, I dunno. I’ll have to try to quantify that later.

I apologize that I did not make any notes about smoothness when manually cycling. I had to clear a few stoppages, and hand cycle many times, but I can’t recall noticing anything unusual.


I would never have guessed that a rifle lower would require less gas than a carbine. Interesting stuff. Thank you for posting that!

This may have something to do with why rifle lowers feel different than carbine lowers. You’re welcome; thanks for reading.

Unfortunately, I found this data collecting exercise enjoyable, but disappointing. I don’t think that the gas block I chose adjusts with quite enough precision to really compare the gas requirements between these different springs as well as I’d hoped. I may add tests later with different buffers; not certain yet.