AR-15 advise

marek13

Hello I am planning to build an Ar15 and I was looking at:
1) A DPMS 24" SS barrel assembly 1/9 twist
2) A silver or black Diamond series lower assembly (with 2 stage trigger)

Now what do you think of this configuration. The barrel assembly cost me 350(with bolt and charging handle). Now how much do the diamond series receivers assy go for? I saw one for 400. Is this good?
And I guess my question is;are the diamond receivers good or should I go with the Bushmaster lower that I can but for 300

gap1916

What do you plan to do with the rifle? This will help in determining what your need will be.

Greg

marek13

Mostly target practice and occasional varmint hunt.

fort_knox

I think Gap1916 makes a very good point. I went along on a Prairie Dog safari with some veteran "black tail" hunters a couple of years ago. I brought along an Armalite w/a 20" 1/9" twist barrel, a 2-stage NM trigger group, and a sufficient scope to reach out there some. I didn't do very well. I found through some research that my 1/9" twist was intended for heavier projectiles than I was using. The "ballistic tip" projectiles I was using in the high 40 to low 50 grain weight were much more suited for a slower twist barrel, such as the 1/12" that was used on earlier AR's...when 55 grain projectiles were standard. Apparently the military had went with a "faster" 1/9" twist rate when they increased the NATO standard projectile to the heavier 62 grain M855...for better accuracy. So, it is my understanding that the faster twist adversely effects the accuracy of the lighter projectiles that it was not designed for. Then again, maybe it was the shooter?

drobs

Marek,

That's the 1st I've heard of a black diamond receiver. Looks like there is one for sale here on GB for like you said, around $400. My only thoughts might be, you could save a few $$ by going with a Rock River lower with a national match trigger. For example:
http://www.gunbroker.com/auction/ViewItem.asp?Item=9187627



Regards,

B-man308

Marek,
Fort knox hit the nail on the head with the twists. I worked on / built M16's for uncle Sam for 9 years. The bullet you want to shoot should determine your twist 1/12=55gr 1/9=62gr and 1/7=69gr. Have seen some 1/10 twists that do a fair job as a happy medium, but to tweek out subMOA with them you almost have to reload. As far as Barrel material; we found black bores the most accurate, chrome lined the most durable and stainless to be the happy medium (I like stainless too). Just make sure you go with a reputable manufacturer. DPMS is fine and so is Bushmaster. Different lowers had very little effect on accuracy - tried large pin, small pin, Colt, DPMS, Bushmaster, Bohica and others - Tight is nice and usually more desirable, but had some real shooters that rattled around like a BB in a boxcar. It was the upper and barrel that made the difference in the end. If it were me, and it has been in the past, I'd save the money on the lower and put it into the Barrel, and a good match trigger. You won't be disappointed

B-Man308 - Former USAF Combat Arms Instructor, NRA Life member

marek13

guys thanks soooo much for all the info.I think I will take your advise and think what kind of twist do i wanna go with and that worry about the lower part
thanks again Mark[:D][:D][:D]

rordog

Gentlemen, a 1/9 twist shoots 55gr. bullets most accurately! I just don't see how you can over-stabilize a bullet. The only problem you could have with a fast twist barrel is the bullet coming apart due to the high RPM as it comes out of the bore, and then you won't see any bulletholes on the target. If you were shooting 69's out of a 1/12, and not getting a group, it would be a twist problem. If you can't get a 50gr. BT to shoot out of a 1/9, then something else is wrong. I have shot to many 55gr-1/9 and 52gr-1/7 to believe otherwise.

fort_knox

Over-stabilizing a bullet doesn't seem to make any common sense... intuitively, I agree. In my case; as I said: "it could be the shooter." I'm not putting all the blame on too light of a projectile and a too fast rifling twist. I've bought too many new golf clubs thinking they were the problem! I recall that it was so windy on the prairie many days that it rocked our 4-wheelers around too much to get much use out of them as bench rests. It also didn't help that we couldn't get much closer than 150-300 yards from something the size of a small rabbit either.

The stuff I posted was from some resources I referred to at that time and took as said...w/o a scientific explanation. I think "www.AR-15.com" may have had some specifications posted regarding bullet weight and barrel twist...and some other resources I don't recall at the moment. Anyway, you got me thinking about it some more...so I did a search on two or three topics.

There is a formula called the "Greenhill Formula" that is used to determine "ideal" twist rates for different bullet weights/types and other conditions that are pertinent to accuracy and stability. Three things determine "bullet stability": 1.) "It must be statically stable" 2.) "It must be dynamically stable" 3.) "It has to be tractable."

The "Tractable" component appears to apply to the phenomenon of "over-stabilization." Quote: "Over-stabilization means that the bullet rotates too fast and becomes incapable of following the bending trajectory, as it's longitudinal axis tends to keep it's direction in space. This effect is often observed for high-angle shooting, but is of minor interest in normal shooting situations." You can find this and an abundance of other information regarding this at: "http://www.nennstiel-ruprecht.de/bullfly/stab.htm"

Also, just do a search on "Barrel Twist" and "Over-Stabilization"... these two got me to the site I referred to in short order. Thanks for the inspiration! Take care.

Boomerang

X-ring - Fort Knox,

This is what I have come to understand about the over stabilization phenom. It is also my understanding that the overstabilization phenom will have its greatest effect on accuracy at extreme distances. For the .223 this would be beyond 500 to 600 meters.

The 1-9 twist is probably the best compromise for most bullet weights commercially available. If you are going to use heavy bullets (80 grainers), you will not only be hand loading them, you will also have to hand feed them into the rifle due to their length. The 1-7 / 1-8 twist barrels are best for this weight bullet.

Boomer

"Success is to be measured not so much by the position that one has reached in life as it is by the obstacles which one has overcome while trying to succeed."

NRA Life Member

B-man308

If you want to see a very elementary example of stabilization, take a top(the toy kind) and spin it as hard as you can. It will wobble all over at first - it is "overstabilized" at this point. It will continue to slow to a speed where it will sit virtually still while spinning - it is now stable. As it slows down more it will start to wobble again - it is "understabilized" at this point. When you start talking about 3000+ FPS - anytime the projectile isn't stable in flight, and the winds don't hit dead-on center on the nose of the projectile, it is going to blow the projectile slightly off course and have an effect on it's accuracy.

B-Man308 - Former USAF Combat Arms Instructor, NRA Life member