I will be building an 18” heavy barrel rifle length upper. I will be putting it on a lower with a Carbine length buffer tube. I have stock Carbine buffet and spring to start. The BCG is a Left hand stag which I assume to be comparable weight to 15 BCG. I don’t think they are M16.

What does arifle length gas system with a short post port dwell time do when cycling? Will I need different buffers/spring?

Will the short post port dwell time reduce the impulse compared to a 20”?

Am I overthinking this?

I have heard it said that the gas impulse unlocks the bolt and then recoil takes over the cycling. Is this somewhat true?

quote:

Am I overthinking this?

Yes, you should be fine.
However, as with any new installation test it for malfunctions, etc.
If you have any cycling issues then make adjustments at that time....if any.

What do you mean by "short post?" Do you mean a low profile gas block? If so, it makes no difference.

A properly spec'd rifle length gas system (not oversized gas port, etc) will have the softest recoil impulse of all standard gas systems. Your Carbine buffer will be ~3.0 oz. A typical rifle buffer will be ~5.0 oz. The closest carbine-sized buffer will be an H2 coming in a 4.7 oz.

Can't really tell over the net. Gas port size and ammo get a vote as well.

I'd start with what you have and see how it cycles with the ammo you are planning to use. If it works reliably, carry on.

If you see crazy ejection patterns, malfunctions or unusual operations you'll want to swap in a different buffer, probably a heavier one.

FWIW BCM ships their 18" rifle gas system rifles with an H buffer:
Link

Also of intrest, the H6 buffer (Army) and A5 buffer/stock tube (USMC) were both developed to allow a military M16A2/A4 to function reliably with a collapsible stock. Neither saw much if any military use.

quote:

Originally posted by LDD:
What do you mean by "short post?" Do you mean a low profile gas block? If so, it makes no difference.

A properly spec'd rifle length gas system (not oversized gas port, etc) will have the softest recoil impulse of all standard gas systems. Your Carbine buffer will be ~3.0 oz. A typical rifle buffer will be ~5.0 oz. The closest carbine-sized buffer will be an H2 coming in a 4.7 oz.

Short post dwell time. The time that the bullet is still in the barrel after it passes the gas port. That is the only amount of time that pressure is sent to the gas key. An 18” rifle length barrel will have a shorter post port dwell time than a typical 20”.

Most 18” barrels that I know of have a mid length gas system.

You're not losing that much dwell time by chopping 2" from the far end of the barrel.

If your rifle doesn't have enough energy to cycle the action with a carbine weight buffer, you're in trouble because that means your only option is to have someone custom-machine lightening cuts into your LH bolt carrier.

I think you'll be fine. If anything, you may be slightly under weighted.

I have an 18" rifle-length AR. It uses a standard carbine buffer, spring, and tube, and it cycles just fine.

The DD V7 Pro and V11 Pro models are built nearly identical to your envisioned build, as far as they're 18" barrels with rifle length gas systems and normal carbine buffers and buffer springs.

I have an 18" barrel with rifle-length gas system. Originally had a rifle buffer, now has a carbine buffer. I installed an H or H2 weight, which helped. This AR has cycle flawless for 3,500 -ish rounds, with factory ammo (both 223 and 5.56) from 40 to 77 grains. I now shoot it suppressed all the time -- it's a little over gassed now.

You should be fine. 18" is well within the envelope for rifle gas. 17 is ok. 16 is marginal.

quote:

Originally posted by LDD:
You're not losing that much dwell time by chopping 2" from the far end of the barrel.

If your rifle doesn't have enough energy to cycle the action with a carbine weight buffer, you're in trouble because that means your only option is to have someone custom-machine lightening cuts into your LH bolt carrier.

I think you'll be fine. If anything, you may be slightly under weighted.

I highly doubt it will be necessary, but he could remove a few of the steel weights from his carbine buffer and replace them with delrin spacers. He would lose the dead blow effect of the floating weights but that is generally only a factor if shooting full auto.