Does anyone know why they don't offer a 1/7 twist in their stainless barrels? It seems common to not offer SS barrels in 1/7. I have seen 1/7.7
Printable View
Does anyone know why they don't offer a 1/7 twist in their stainless barrels? It seems common to not offer SS barrels in 1/7. I have seen 1/7.7
SS barrels are 1x8, what does 1x7.7 offer than 1x8 does not.....just curious. I find that the 1x8 BCM SS410 barrel I have pushes 75gr and 77gr bullets about 65-75fps faster than my 1x7 twist BCM chrome lined barrels.
I believe most barrel makers that make stainless barrels find that 1x8 is more accurate and faster for ammo between 55gr to 77gr vs. 1x7. Remember 1x7 was only used to stabilize the M856 tracer round (64gr.) that the military uses with the M249, it's this tracers projectile which is longer than a 77gr Sierra MatchKing even though it's a lot lighter. Length requires faster twist vs. weight.
This is also true with Barnes TSX bullets for example a 62gr TSX is much longer than a 62gr SS109 projectile from a M855 round.
Remember that while they list it as 1:8, it could be off by as much as a half inch in either direction. It's very likely the 1:8 is actually a 1:7.7. The 1:8 twist is proving to be very versatile for the 5.56
So I guess what your saying is that a 1/7 twist has no "real world" advantage being that most people do not fire tracers?
I found this website after an 8-9 year shooting hiatus and from what you read here 1/7 twist is highly desirable. Back then, a 1/8 was the rage with NM/CMP crowd.
Also, I read in "The Complete Guide to AR-15 Accuracy" (the Accuracy Speaks guy)years ago that you can't overspin a bullet (except for ultra light varmint bullets) so I figured "why not 1/7?".
I mentioned Bravo Co. because I was interested in their barrel for a future CMP/NM build but also realized they primarily cater to the "tactical" and "m4carbine.net" crowd. I was surprised they wouldn't offer a 1/7 twist. I still feel there is a piece of the story missing here. My gut is telling me it is material or equipment related.
To be honest, the only reason I was looking at the BCM barrel was that you can get it in 20" and in black AND it seems accurate enough for CMP/NRA High Power. I hate the look of SS on an AR.;)
When companies like this http://www.whiteoakarmament.com/ offer them it make it even more perplexing why Bravo doesn't.
You can blame me for the BCM SS barrels being 1/8 twist. I talked Paul into it. Why? Because 1/8 is really the best "ALL AROUND" option there is and I would love to see more barrel manufacturers use this twist rate.
C4
Aha! I knew there was something to this. ;)
Now if you could just talk him into incorporating those 20" barrels into CMP/NM uppers he could open himself up to another part of the market.
There are a ton of CMP types that figure if RRA and BM make decent CMP rifles then their tactical rifles are GTG. Some CMP/NM traffic may help his carbine business as well.....
I've posted this before but it's not the answer most people are looking for. But for completeness here is some material to help you with twist rate http://www.jbmballistics.com/ballist...tability_1.pdf
-.-
So could we agree that 1/8 should become the industry standard?
There's really no conspiracy here. As Grant stated, the 1/8 offers more versatility and is an excellent choice by BCM.
It seems that most shooters interested in a SS barrel from a company like BCM are going to be shooting longer or heavier bullets. As long as the bullet can maintain its structural integrity, the only argument I can imagine against a 1/7 twist would be barrel wear.....if that is even a legitimate concern.
Would the industry be served well by only offering 1/7 and 1/12 (for varmint guys) and skipping 1/8 and 1/9?
NOTE: I am not a ballistics expert and I am just speculating.
Why should we ask for an "industry standard" when it comes to twist? Let the manufactures offer what they will and the market will sort itself through real world performance.
The tighter the twist, the more energy needed to spin the bullet and the greater the throat erosion along with an increase in pressure and loss of velocity. If the twist is too tight, the bullet will skid. Remember, TANSTAAFL.
Sierra has done extensive real world testing. They took barrels of various twists and tested them to see how twist affects the in-flight ballistic co-efficient of their 69 gr HPBT MatchKing bullet. Can twist rate change the in-flight BC of a bullet? Yes. How? If the bullet is a bit unstable, the nose of the bullet moves around on it's axis, what Sierra calls "coning". This increases drag.
The ideal twist is one that stabilizes the bullet without loss of velocity or increased pressure and wear. Sierra tested twists from 1:7 to 1:10 and 1:12 (they did not have a 1:11 barrel to test).
They got satisfactory results with twists as slow as 1:10, though at that point they saw deterioration of BC with the 1:10. The 1:12 had a significant loss of BC.
Their conclusion is that all bullets have some coning. The tighter the twist, the less coning there will be.
What is interesting to note is shots from the 1:7 twist and 1:8 twist barrels had the same BC but the 1:8 had 200 fps more velocity. The 1:7 had less Standard Deviation and Extreme Spread.
You can read more in the Sierra Reloading Manual, Ed. V, 5th Printing starting on page 1019
To anyone who has a SS upper is there anything you do not like about it? I was considering a bfh upper or the ss upper, each in mid length. They are around the same price.
Regardless of what you do with it is there anything you don't like about it?
It would be for general recreational use, no professional use.
I was looking at an ion-bond barrel from them. I am assuming that the the ion-bond is on the inside of the barrel as well. (?)
If that is the case, I am wondering if accuracy is affected as the ion-bond would wear.