I’m trying to choose between two ARs that are identical, except for a barrel length difference of 16 or 18 inches. The gas block is in the same location relative to the chamber end, at the midlength position. This means the length of barrel in front of the gas block is 2 inches longer on the 18 inch barrel than on the 16 inch barrel. In turn, this means the dwell time (bullet between gas block and muzzle) is correspondingly longer in the 18 inch barrel.
I’ve seen charts that say the dwell time increases from 0.018 to 0.023 seconds between a 16 in and 18 in barrel with a midlength gas system, using 55 gr ammo. (I expect to use 77 gr ammo, so I expect the dwell time to be a bit longer.)
My theoretical intuition tells me that the longer dwell time of the 18 in barrel should result in less accuracy than the shorter dwell time of the 16 in barrel, since all else is equal. I notice that the SPR design uses an 18 in barrel with an intermediate length gas block, which puts the gas block further forward than the midlength system.
So my question to AR technical gurus, and those who have experience with the situation above: Are my concerns above valid? Is the accuracy difference measurable (and consistent) in bench rest groups with heavy match ammo? Does anyone have examples of bench rest results that show a clear difference (or clearly no difference)?
I’m trying to learn the results from expert experiences, rather than try to buy both and figure it out on my own. Thanks!
I cannot address the dwell time issue, but can offer to point out that the US DOD chose 18 inch barrels for their 5.56 SPRs, and they are seeking accurately placed shots to 800 meters.
The Mk12 SPRs have an 18" barrel with an intermediate length gas system (between mid length and rifle length), so the dwell time on the SPR 18" barrel with intermediate gas system should be similar to that of a 20" barrel with rifle length gas system or 16" barrel with midlength gas system.
If I get an 18" barrel with midlength gas system, I’ll be increasing the dwell time compared to a 18" barrel with intermediate length gas system. I’m wondering if/how much this will reduce accuracy, and what the net effect is compared to a 16" barrel with the same midlength gas system.
I don’t know about accuracy shift but somewhere USMC03 added a post about barrel velocities and he got in to dwell times of different barrel lengths.
He stated that the 16" barrels with a carbine length gas system created a longer dwell time than a 14.5" and would create a rifle with more violent recoil. So I would guess that you could be looking at more recoil impulse with the 18" over the 16".
Found this on USMC03 web site. Carbine vs. Mid-length Doesn’t compare 18" vs. 16" but is a good read for people concerned about gas system lengths.
My LMT .308 has a carbine length gas system, I really wish they would have gone with mid length, but we will see if they release barrels with a mid length.
The Mk12 uses a RIFLE length gas system not intermediate. The Noveske 18" SPR barrel uses an intermediate length system but its not the correct mil-spec barrel. If you tried to use a Noveske barrel on a clone build the gas block would be under the handguards.
I’ve seen charts that say the dwell time increases from 0.018 to 0.023 seconds between a 16 in and 18 in barrel with a midlength gas system, using 55 gr ammo.
For starters, your numbers are WAY off. It takes a 55 grain bullet less than 1 millisecond, that’s 0.001 seconds, to travel the entire length of an 18” barrel. It takes less than 0.9 milliseconds, 0.0009 seconds, for that 55 grain bullet to travel the entire length of a 16” barrel. The difference between the dwell times of a 16” barrel and an 18” barrel that both have a mid-length gas system will be approximately 0.06 milliseconds; that’s 0.00006 seconds.
55 grain bullet from 18” barrel
55 grain bullet from 16” barrel
Originally posted by Sundo:
My theoretical intuition tells me that the longer dwell time of the 18 in barrel should result in less accuracy than the shorter dwell time of the 16 in barrel, since all else is equal.
A. I’ve never seen a single study showing a causal relationship between an increase in the amount of time that it takes the bullet to travel from the gas port to the muzzle and a decrease in accuracy in an AR-15.
Rifle length gas system does that but more so than the intermediate system.
The rifle length gas system and rifle length buffer system makes for the softest cycling system. [The way it was originally designed]
On the 5.56 MRP I think they have 2 different gas system lengths carbine and mid length if I remember correctly. The .308 MRP use of a carbine gas system I don’t know for fact but I imagine that LMT has there Bolt carrier correctly ported and timed for use in the carbine gas system.
No, it’s milder than a carbine but is still 5.5 inches shorter than the rifle. Lot’s more felt blast. Recoil on any is not an issue. It’s muzzle rise and blast that effects your shooting with an 5.56 AR system.
