Effects of dwell time on reliability with 14.5" vs 16" middy

[Skyfire1201]

Hi all,

Apologies in advance if this topics has been debated to death, but I've searched the forum and looked for a definitive answer on this question, as well as reading through AR15 operation (http://ar15barrels.com/prod/operation.shtml) article and still haven't quite found the answer.

The question is: how does dwell time affect operations on a 14.5" and 16" middy, and which length is better from a reliability perspective on a midlength setup.

The Ar15 operations article listed above concluded that optimal dwell time is 0.200ms, which means 16" on a middy or 14.5" on a carbine. What I don't quite get is why 0.2ms in particular, other than it's the "standard" on 20" rifle, 16" middy and 14.5" carbines? It seems to me that as long as dwell time is not too long as to hold the spent casing in the chamber when the bolt unlocks and starts rearward movement, shorter dwell time should not affect normal operations. Am I missing something here?

Reason I'm asking is I'm planning on building a lightweight carbine, and trying to decide between 14.5" or 16" barrel. 14.5" would be nice since it is shorter and even lessor weight, but IMO wouldn't be worth it if it affects reliability in anyway. BTW, I plan to go with either DD or BCM for both upper and lower, assuming both of their middy upper being comparable quality (optimal gas port size etc).

Thanks in advance!

[bkb0000]

without getting into it too much (which has been done, to death, as you fear), suffice it to say that the 14.5 midlength has been out and used for a couple years now, and longer on a smaller scale, and there have been no problems reported. we haven't seen any increase in malfunctions around here, and no individual gunsmith types have reported seeing any increase in malfunctions. therefor, a well built 14.5 can be assumed to be just as reliable as any other vetted system. my own 14.5 midlength experiences have been the same- no change, in reliability or performance... i'm actually pretty underwhelmed, and will likely go back to carbine-length 14.5s in the future, for weight savings.

build in peace, and with confidence.

[jmart]

I don't know what the minimum dwell time required is for a given port diameter, but if the port gets too small, you won't have sufficient gas volume routed to the BCG to cycle the action. That's why SBRs have larger ports -- to provide sufficient gas given a short amount of dwell time. It's an weapons design/integration issue, you need to to come up with the proper port size and dwell time combo to provide sufficient gas volume for weapons operation.

[87GN]

The 18" rifle gas system has less dwell time than the 14.5" midlength, and the former has seen combat in the form of the various Mk12s for quite some time.

[BSWilson]

The 18" rifle gas system has less dwell time than the 14.5" midlength, and the former has seen combat in the form of the various Mk12s for quite some time.

But not without issues in colder climates, and not without opening the gas port up a bit, if I recall correctly.

[87GN]

But not without issues in colder climates, and not without opening the gas port up a bit, if I recall correctly.

Well, I never said that they use the same gas port as the 20".

I've heard the "issues in colder climates" thing but have never experienced it myself or seen hard evidence of it.

[JDW67]

without getting into it too much (which has been done, to death, as you fear), suffice it to say that the 14.5 midlength has been out and used for a couple years now, and longer on a smaller scale, and there have been no problems reported. we haven't seen any increase in malfunctions around here, and no individual gunsmith types have reported seeing any increase in malfunctions. therefor, a well built 14.5 can be assumed to be just as reliable as any other vetted system. my own 14.5 midlength experiences have been the same- no change, in reliability or performance... i'm actually pretty underwhelmed, and will likely go back to carbine-length 14.5s in the future, for weight savings.

build in peace, and with confidence.

I have to agree with bkb. I was told middies were god's gift to the civies, but I can't feel or see any discernable differences between middies and carbines. I do like the extra 2" of rail space, but other than that...

[ATF Agent]

I have to agree with bkb. I was told middies were god's gift to the civies, but I can't feel or see any discernable differences between middies and carbines. I do like the extra 2" of rail space, but other than that...

Are you referring to 14.5" carbines vs 16" middies, or 16" middies vs 16" carbines, or 14.5 carbine vs 14.5 middy?

[JDW67]

Are you referring to 14.5" carbines vs 16" middies, or 16" middies vs 16" carbines, or 14.5 carbine vs 14.5 middy? You have to compare identical barrel lengths for there to be a difference in felt recoil.

I have a 14.5 middy and a 14.5 carbine. I honestly can not feel any difference between the two while shooting.

[ATF Agent]

I have a 14.5 middy and a 14.5 carbine. I honestly can not feel any difference between the two while shooting.

Forget what I said about identical barrel lengths, that is not true now that I have thought more about it. I think there would definitely be a difference between a 16" carbine and a 14.5 midlength. I guess what I meant to say was that for a fair comparison, they should be identical barrel lengths.

[bkb0000]

it's been many a long year since i've even fired a 16" carbine... but i would always chose a 16" midlength over a 16" carbine. it's just way too much excess gas- the very inefficiency of it bugs me, if nothing else. all that extra pressure has got to be significantly more abusive than it needs to be, though there just really isn't any compiled data to support the theory.

and perceived recoil aside: though, again, in theory, that should be "improved," i cant say as i ever noticed a difference. ammo and muzzle device will have a lot more to do with how your weapon handles recoil than anything else. after that, it's all you.