6.8 questions

[Bubba FAL]

FWIW, I believe that you're relying far too much on theory and not enough on the impirical evidence.

I've been down the SAAMI 1:10 and SPCII 1:11 path personally. My SPCII 1:11 6.8 handles loads far hotter than the SAAMI 1:10 6.8. These are not excessive loads either - if I wanted any kind of brass life whatsoever, I could not fire loads heavier than 26.5gr of H332 in the SAAMI gun. This isn't much over a starting load according to published data. (BTW - I've gauged the bores on both my 6.8s and neither is "undersized".)

Since reaming the SAAMI chamber to SPCII, I can run loads at the published maximum in either gun.

If you've been on 68forums and read the stickies, you should have seen the pressure test reports. Yes, there are other factors involved such as land to groove ratio, etc., but the fact remains, you will severely limit your options if you use a fast-twist barrel on a 6.8.

The longer .270 bullets are not suitable for loading in the 6.8 case, so faster twists beyond 1:10 are not necessary.

Doc Roberts has done plenty of testing of the 6.8 for tactical use and his advice should be heeded.

[rsilvers]

Agreed that twists faster than 1:10 are rarely necessary. I would best compare it to 1:8 on a 5.56mm. If you have a 1:8 barrel you can shoot just about anything. In fact, 1:8 probability would have been a great choice as the 5.56mm standard.

I saw the pressure tests, but they were strain gauge reports mapped into pressure. I enjoyed reading it as I own a Pressure Trace-II - the same unit they used. It does better for relative data on the same barrel and is not reliable for absolute results between barrels.

The best way to know if 1:10 vs 1:11 makes a difference is to have two pressure barrels made with the same reamer and from the same blank, and compare them with conformal transducers. It has been done with 5.56mm 1:7 compared to 1:12, and the reports range from no measurable difference to a minimal difference.

If you want lower pressure for any given twist rate, try to have GO+0.002 headspace rather than GO+0.000. Make sure the reamer is 0.0015 over minimum chamber dimensions. Use an SPC-II chamber as was meant to be. Ensure that your bore cross-sectional area is not at SAAMI minimum or less. If you do all of that, there will be no pressure problems.

[PatEgan]

You are right, most are.

But really, when I see higher velocity on certain barrels, it makes we want to measure the chambers to know their dimensions. That is the only way I can hope to know the effect of the other barrel parameters.

Fair enough. I think you'll like what you see on 68forums regarding the benefits of lower pressure/higher velocity due to SPEC II chambers, slower twists and fewer grooves (less friction) in the barrel.

The stickies are particularly helpful, and I'd recommend the stickies in the Reloading forum at the same site.
Pat

[constructor]

Silvers has already been to 68 forums, tried to tell us an 8 twist barrel was best for the 6.8. It didn't go over so well.

[rsilvers]

I may settle somewhere in the 1:7 to 1:9 range ultimately, but I am cool with 1:10 to 1:12 for most uses until I get the results of physical testing.

[LONGBOWAH]

It has not been proven that 1:11 or 1:12 twist has measurably less pressure than 1:10 twist, nor would I expect it to. Why would you want to give up bullet stability under extreme conditions of cold weather and when using a sound suppressor and give up the terminal effects benefit of a faster twist with certain bullets when there is no significant pressure difference?

Every SAAMI 6.8 barrel is able to handle a 78,500 psi proof round so really a SAAMI chamber can handle enough pressure for any reasonable factory load - which should all be under 60,000 psi anyway.

Both LWRC and LMT (which I consider to be top end OEMs) have both solved the pressure "problem" with 1:10 twist rates. LWRC in particular thinks that the extra rotational velocity is especially important with shorter barrels.

The question, IMO, is if you are going to handload to get that last FPS out of your barrel, then yes, slower twist, custom chamber, etc. is the way to go.

If being able to shoot SSA TAC loads (without excessive pressure) is "good enough", then LWRC and LMT are two factory alternatives.