5.56 - 16" vs. 20" ballistics

[MMH]

How much does the velocity drop in a 16" barrel vs. a 20" barrel? I can only afford to build one rifle right now and am leaning towards a 16" middy w/ collapsible stock. Clearly this is a better choice for HD, etc. At the range, the 20" w/ bull barrel would be superior. I know that I'm trying to get one rifle to do it all, so help me pick the right one!

[Legion_Prime]

Well that depends how long the range you go to is. Most likely there will not be much of a difference at the ranges most ranges are limited to. I'd go with the 16" personally.

[GRIZ]

I chronographed 55 gr federal FMJ:

3140 fps in a 20"
2850 fps in a 16"

RP & WW were also pretty much the same plus or minus 20-30 fps.

The 16" is as accurate as the 20" slightly different sight adjustemnts to compensate for bullet drop at range.

[mrnyman]

The rule of thumb is 50 fps per inch of barrel loss, 4 inches = 200 fps
200 fps is quite insignificant in a home defense situation.

Rule of thumb is ; use heavier bullets in the short barrels, = better down range ballistics.

Yeah so you are looking at 200 feet per second loss.

nyman

[Tomcat088]

What others are saying is pretty much correct, you're probably looking at around a 200 fps loss, maybe a bit more depending on the loads, etc. I think it all depends on what you want to do with the rifle. I personally think that 16" is a little short, but I'm more accustomed to rifles that are longer (my bolt rifle build has a 26" barrel on it). I personally don't think that 20" is all that long.

Remember that barrel length doesn't just affect velocity, it can have effects on other things. Since the sight isn't on the "end of the barrel", you're not losing in terms of sight radius, but there's other things to consider. Barrel length and contour will affect how the rifle balances and recoils. Due to the nature of the AR and the caliber, you won't see much of a difference in barrel rise, BUT there might be a little bit of difference in balance. I'm not saying the shorter will balance better or worse, it's a personal thing, and you might want to handle the firearms somewhere else even if you're not going to by them at the store, to get an idea of how they feel. If you're going with a 20" bull, realize that the rifle will probably feel a bit front heavy, if it's not a bull barrel, this probably won't be the case. Just consider that you're not only changing the length and losing velocity, but just how the rifle handles, balances, etc. Either barrel length will be more than sufficient in terms of kinetic energy and velocity for "conventional distances". The question you need to ask if are you trying to build a "conventional" rifle now? Are you wanting the 20" bull becuase you might want to do something later? Will you possibly be able to afford the different barrel for your AR later? What is the true purpose and main use for this rifle? That's what you need to weigh in your head, and make your choices accordingly.

[Asmodeus6]

The rule of thumb is 50 fps per inch of barrel loss, 4 inches = 200 fps
200 fps is quite insignificant in a home defense situation.

Rule of thumb is ; use heavier bullets in the short barrels, = better down range ballistics.

Yeah so you are looking at 200 feet per second loss.

nyman

Are these numbers specific to 5.56 or anything?