18" or 16"?

[Redneck19]

Hey guys:
I hope I'm not beating a dead horse here, I couldn't find another thread here that covered this, so here goes:

I'm looking at rebuilding my current carbine upper with at least a mid length gas check. I like the 16" but I'm trying to decide if I should go with a true mid length with an 18" barrel.
What are the advantages of a mid length over a carbine? Does it extend the range out much beyond 500 meters?
It seems like most military balistics charts, etc. are based on the M16 or the M4. I haven't really found anything on the mid length "recce" guns, so any info would be appreciated.

[Belmont31R]

A 16" is fine out to 500.


RE velocity 16" is about 100FPS faster than a 14.5".



I would recommend if you want to shoot long distance (for an AR) you shoot 5.56 pressure stuff. Its a lot flatter shooting than .223 stuff especially the real weak brands. Im only 29 clicks down (.10 mil clicks) at 500 with 5.56 pressure ammo yet 223 is down around 40. 556 pressure ammo should be about ~300FPS faster than .223 for similar load types.


I would say get the 18" if you plan on shooting it like a DMR/SPR...a variable scope, bipod, ect. If you're going to use something like an ACOG (4X or less) I would get the 16".


Here is both an 18" and 16". Obviously they have pretty different roles but have some over lap.

[Failure2Stop]

You have some naming conventions off, so here is what those words mean:

1- Gas Port (not gas check). It is the hole in the barrel that diverts gas pressure from behind the projectile back through the gas tube to the BCG.

2- Mid-Length refers to the Gas Port location in relation to the chamber. Is is almost (but not quite) standardized at this point. A carbine gas system will have the port at about 7.5 inches from the chamber, middie will be about 9.5 inches, and rifle will be about 13 inches. The intermediate gas system, which was developed for 18" barrels, is about 11.5 inches. While some of these have common barrel lengths associated with them, there isn't a fixed barrel length that they must use. There are some 18" barrels with rifle length gas systems, others with intermediate gas systems, and others with mid-length systems. There are mid-length 16" and 14.5" barrels. Pretty much anything under 12.5 is going to have a carbine length gas system, unless it is around 7" in which case it will have a pistol gas system (about 4.5").
*NOTE*- My numbers are approximates, I don't have the exact distances handy, but they are close enough for practical purposes. As far as mid-lengths are concerned, most manufacturers use the sandard set my ArmaLite, but KAC uses a slightly longer distance to port.

3- You will increase velocity by adding barrel, gas port location will have no appreciable effect. Adding velocity will not necessarily increase precision.

4- There is a ton of data for 16" barrels, drop into the Ammo and Reloading Sub-Forum and check it out. Molon has shared lots of good data on the subject, and there are plenty of others that have done the same. It is generally harder to find data for non-mil ammo for 14.5 barrels because there aren't many people willing to deal with the NFA hoops to only lose 1.5" of barrel length.

[Redneck19]

Ok WOW!

Thanks for the education Failure2Stop.
I am specifically interested in the "middie" design and assumed that the 9" rail attributed to this type of gas port was predominantly used on the 18" barrels (or so I have heard).
Does that sound right?
So for a mid length upper, like the BCM Mid length light weight, would the handguard be 9" or ???

Belmont31R:
My current upper is a 1:7 and I was shooting 55 gr. .223 and just recently swapped to 62 gr. Does that matter in the comparison between .223 and 5.56?

[Belmont31R]

Belmont31R:
My current upper is a 1:7 and I was shooting 55 gr. .223 and just recently swapped to 62 gr. Does that matter in the comparison between .223 and 5.56?

Yes it still matters.


I would also point out it doesn't matter too much if you know your dope, and can adjust for it.


Based on what Ive seen you want to be shooting some kind of match ammo past 300 yards. Ive shot quite a bit of cheap FMJ out to 500+, and it just doesn't have the consistency you need to make reliable hits. You can shoot a 30 rd mag full of cheap FMJ ammo, and maybe score hits 1/3rd of the time or shoot quality match ammo, and score hits around 90-95% of the time. The best group Ive ever shot with FMJ at 100 was a little under 2", and at 500 that equals a 10" group. Add wind, mirage, ect to the mix, and thats why you would have a tough time hitting reliable on a target at 500. With a heavy match bullet wind effect is lessened, 5.56 so it gets their quicker (less time for wind to effect the bullet, increased energy & flatter shooting), and a match grade bullet so your rounds are not peppering around the target but hitting it instead.

Another example of the difference between .223 and 5.56 for my 18" gun. With Mk262 rounds (5.56 pressure with a 77 grain SMK) at 300 Im down 11 clicks (.10 mil @ 100) but with Hornady Training rounds in .223 with a 75 grain BTHP bullet Im down 18 clicks which is close to being half as much drop. They are both very accurate rounds for me but its hard to argue against such a flatter shooting round.

[Cameron]

It really doesn't matter if you get a 16" or an 18" now, as you will want the other in short order.

16"


18"



However, I would get a 16" if I only had one...

Cam