View Full Version : 6.8 SPC 1 / 9.5 twist J&T upper question.
10-26-2009, 01:08 AM
I have a J&T upper with a 16 inch chrome lined barrel that I bought at a Gun Show 2 1/2 years ago and it is a great shooter and I have never had the fist problem. After reading articals and information I am a little confused, most folks talk about the 6.8 having a 1 / 10 twist, even Silver State ammo test their ammo in a 1 /10 twist barrel. My J & T has a 1 / 9.5 rate of twist, does this cause any concerns about ammo selection? Like most shooters I have tried different ammo, The Remington is the worst and my rifle likes the Silver State 110 grain Pro Hu nter. I see that Silver State has other loads that are listed as commercial and others that are Tactical , I am going to stick with rthe comercial load but what would the Tactical load do in a 1 / 9.5 instead of the 1 / 10 twist barrel they tested the loads with. I just want to make sure I understand what potential problems to avoid.
10-26-2009, 05:37 AM
I would avoid using the tacticals as they will most likely cause problems due to excessive pressure in your rifle. The most common issue I would expect to have would be blown primers or split case mouths. If it is a good shooter and you are satisfied with its performance to date with standard ammunition. then keep shooting and possibly look into the reamer to open yours to spc II but if you already know what your rifle can handle and what it digests well then you should be fine.
10-26-2009, 09:26 AM
Off the shelf commercial ammunition and similar handloads are fine with your barrel. Your barrel specs are of the older generation. The industry has moved away from those specifications to slower twists and better chambers which significantly reduce operating pressures and allow for increased velocities.
SSA's minimum requirements for the Tactical loads are SPC II chamber and 1:10 or slower twists like 1:11, or 1:12. You likely have the SAAMI (1st generation) chamber which does not qualify for the Tactical loads and your twist is too fast for the Tactical loads. These loads while perfectly safe in the newer generation barrels, would create excessive pressure in your barrel.
One thing you can do to improve your barrel is have the chamber reamed to SPC II specs. All this does is add an additional .005" to the leade which makes all the difference in the world in pressure reduction. Reduce the pressure and your rifle components like your bolt will last longer. This can be done inexpensively and is worth the money.
So shoot all the off the shelf commercial ammo and comparable reloads you want to. Look at converting your chamber and that will lower pressure and give you some velocity gains without rebarreling your rifle.
Later if you wish, you can replace that barrel with one of the newer spec'd barrels like ARP, Titan, Wilson Combat, and Bison Armory sells.
Here's a pressure testing report that answers some of the questions you raised:
10-26-2009, 08:13 PM
Thank you for the information, it helped in my understanding of the evolution of this Caliber. I did call J&T and they said my paticular Upper was made during a transition period between the SSAM1 and the SPCII so the tech rep could not tell me which chamber spec it was. I did do one thing to satisfy my curiousoty just to check and make sure that the round was not touching the rifling. I pulled one bullet dumped the powder and reseated the bullet just inside the case. I then chambered the round slowly and meased the overall length, I repeated this 3 times to insure accuracy. I then measured several rounds out of the same box of ammo to get the Factory seated depth and overall length. The difference between the two is 0.090 inches, so thats the distance the bullet travels before it comes in contact with the rifling. The round I used to check this with is the SSA Pro Hunter110 Gr. The next time I shoot I make sure I check my brass for signs of high pressure. Thank's again for the detailed information.
10-26-2009, 08:27 PM
J&T and their sister company Doublestar produce some solid 5.56 choices. They are just now migrating to the newer 6.8 specs and are a little behind other companies.
You have many good ammo choices for your upper for now. So enjoy shooting it. Keep in mind your 6.8 still runs circles around any 5.56 upper!!
Having the chamber reamed to SPC II will get you some benefits, and it's easy and affordable to have done. As you noticed in the pressure report, the SPC II chamber drops pressure quite a bit.
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