bolt unlock timing

[triggerjerk]

Howdy. First post here. Guess I'll apologize in advance for stupid questions and annoying comments to come. Note I've been shooting for 46 or 8 years (have even done some long range), but have only played AR for 2. Have a weak mind trying to understand a not necessarily simple platform. Anyway:

Ass I've been reading here about gassing issues/cures and general weapon function, I get to wondering:

Would anyone be willing to specifically describe differences in effect (if any) on bolt unlock timing by gas flow control (adj block) vs heavier spring/weight cures? Maybe even relate this to any potential effects on accuracy?

Thanks in advance.

[MistWolf]

In the simplest terms, the lesser the gas flow, the longer it takes to pressurize the expansion chamber in the carrier enough to start it moving. However, as it's residual pressure (pressure in the system after the bullet exits the barrel) the gas has to pressurize the expansion chamber before the pressure drops too far.

If the gas pressurizes the expansion chamber too quickly, the carrier gets moving too soon and unlocks the bolt too early. Gas flow (along with reciprocating mass and spring rate) is a balancing act.

[triggerjerk]

Thanks MW. So, my 16 in, 8 twst, mid lngth Wilson Combat in BCM upper is throwing 2-230 with SpCo blue and H3. If I choose to further tame and fantasize accuracy, I MIGHT be better off with an adj gas block (and lighter spring/buffer) than a heavier spring?

If so or I just want to go adj block, what spring and buffer should I tune to?

It's running fine on chump ammo. Plan to feed it better, soon.... Sorry, for a dummy like me, it's as much about the journey....

[GH41]

And to add to what MistWolf said.. The more respected brands here are over gassed for a reason. Dependability with most ammunition under all conditions. The additional wear due to over gassing is insignificant if you factor in the cost of the ammunition it will take to wear something out. In my opinion there are people regulating gas volume that see benefits from doing it but for the average Joe with a quality rifle the juice isn't worth the squeeze. The exception would be those running suppressors.

[triggerjerk]

Yup, a gun savvy friend has just about convinced me, "Get it running acceptably, shoot it, replace parts, repeat."

On my orig upper, I was 3 engraved cam pins (they prob each could have gone longer), an ejector(was it just the spring?), and gas rings (actually wouldn't support bolt weight) by 2000 rounds (here I added adj block). All in all, these parts were cheep/easy to replace. Gun running well.

But being the type who checks boxes just for peace of mind, I'd be willing to spend the $130 for a block, beat the forend off/back on, and tune.... if it stood a reasonable chance of getting me another 1/4 min accuracy out of this upper.

Guess I oughta shut up, work up some loads and see what I have, butt...

[mack7.62]

I fail to see how adjusting gas flow will improve accuracy.

[triggerjerk]

Not having a clue, I was wondering if premature unlock and any associated leakage with bullet maybe still under influence of pressures in barrel would be detrimental. Also, just for kicks and giggles, I simply wondered what effect the oft discussed "heaviest buffer your rifle will run" would have on timing of unlock. In over my head. Oops.

In retrospect, maybe I should quit trying to understand things beyond my comprehension and shudda just asked, "If one were to install an adjustable gas block on a 16 inch 8 twist middie, what spring and buffer would be good to start with?"

[Clint]

Not having a clue, I was wondering if premature unlock and any associated leakage with bullet maybe still under influence of pressures in barrel would be detrimental. Also, just for kicks and giggles, I simply wondered what effect the oft discussed "heaviest buffer your rifle will run" would have on timing of unlock. In over my head. Oops.

In retrospect, maybe I should quit trying to understand things beyond my comprehension and shudda just asked, "If one were to install an adjustable gas block on a 16 inch 8 twist middie, what spring and buffer would be good to start with?"

Most any combination can be made to run for the conditions of the day.

The best combinations have the widest range of reliability.

An H2 buffer with mill spec spring or extra power spring is a very good starting point.

Gas drive can be adjusted from there to obtain good function.

[MistWolf]

If...I just want to go adj block, what spring and buffer should I tune to?

From another thread-

...I drove out to the desert with my two shorties and different buffers. Testing with the SLR equipped suppressed 11.5" upper showed that the Carbine, H and H2 buffer all ran on the same setting. However, recoil was a tick sharper and felt quicker with the Carbine buffer. Of the three, the H2 felt the softest and slowest.

Next, the SLR upper was tried on the lower with the A5H2 buffer and green spring, it felt even softer than the H2 and almost too slow, until shots were fired rapidly. The combination with the flattest recoil was the SLR upper and A5H2. Next flattest was the same upper with the H2.

The SLR upper was tuned using the lock back check. It was tuned until the round would eject, but not lock back. Opening the gas port one more click resulted in full ejection and lock back. On this day, the SLR gave full function on the same setting with all four buffer types.

The second upper tested is a suppressed 11.5" Colt 6933. It comes from the factory with a gas port that measures about .073". Unsuppressed, it worked fine but felt a little over gassed. With the addition of a suppressor, recoil became uncomfortably sharp. I bought a BRT Micro Port kit. Based on the specs of the AR (11.5" barrel, carbine gas system, A5H2 buffer, shot suppressed only) BRT sent the kit with a .063" micro port. Recoil isn't as soft as the SLR upper but I have no doubt the Micro Ported Colt upper will work even when cold and heavily fouled. When the SLR upper was tried with the H2 buffer, recoil was noticeably sharper.

I didn't collect the brass to see if there was a noticeable reduction in fouling. The grass where I stopped to shoot was 3 - 4 inches thick and made finding the empties challenging.

That the SLR upper fully functioned with all four buffer weights leads me to believe that while the proper buffer weight is important, having the right gas drive is more so.

[triggerjerk]

Wonder was slr originally tuned to carbine or h2? Semi parallel experience: I have one carbine gassed 16in upper tuned to an h with slight extra insurance opening to insure function that locks back fine on h2....

[Lefty223]

Maybe even relate this to any potential effects on accuracy?
See: http://apdmarksmanshipteam.org/blog/got-gas-guide-understanding-ar15-gas-system/

[MistWolf]

Wonder was slr originally tuned to carbine or h2? Semi parallel experience: I have one carbine gassed 16in upper tuned to an h with slight extra insurance opening to insure function that locks back fine on h2....

The SLR upper was originally tuned with the H buffer. It did not matter if the carbine, H or H2 buffer was used, the SLR upper required the same setting to lock back on the last shot.

[triggerjerk]

Apd article sums things up cleanly. First couple of comments afterward interesting as well. Thanks to all for responses.

Sounds like other than reducing gas to stop extractors from ripping off case rims and prematurely destroying c pins, or other extreme overgas function issues we are mostly tuning for recoil comfort and or sight re-acquisition on follow up shots?

[308sako]

I think it comes down to the harmonics induced to the rifle if the BCG is moving too soon vs unlocking at an optimal moment. So much is just theoretical in this sort of discussion. I believe from observation that it is beneficial to have a tuned rifle. How one gets to that point is of course the meat of it all.

[GH41]

Back to the OP's accuracy question.. With an adjustable gas block wide open on a rifle with a light buffer and spring. Worst case scenario.. Right? Shoot a 10 round group. Close the gas block and shoot another 10 round group cycling manually. Throw out the worst 5 shots on each target and measure the 5 remaining. Is there any noticeable difference in the 2 groups? I don't see how there could be. The relationship between the bolt, extension and case doesn't change until the bullet clears the port. Could the residual pressure behind the bullet before it clears the muzzle effect accuracy or Is the bullet long gone before the bolt unlocks? I better shut up. I am making my own head hurt!!

[triggerjerk]

Apologies GH. You sound like a guy who is stuck werking with me (but he needs it).

Was gonna load some and see how current setup performs, but discovered 223 dies rusted to pits. Aaaagh! It snowed an inch here and all the chickens closed the reloading die stores. Aaaagh....