rate of twist ?
I started reloading a TC Contender 7mmTCU any loads I found are for a 10" barrel with a 1 twist in 10 ". I have a 21" barrel with a 1 twist in 11". I shot some Hornady 139's but did not get any decent group. The books also shows loads for 120's and 154's. With my "slower" twist should I go lighter 120 or heavier 154's
0
-
Try the 120gr. 139's are a little long for an 11 inch twist unless you push them up to rifle velocity. 0 -
The general rule is "less bullet, less twist." That usually means a slower twist to stabilize a lighter bullet, and a faster twist to stabilize a heavier one.
As mentioned above, muzzle velocity enters into it as well. What you are trying to achieve is a minimum spin rate (as high as an amazing 180,000 rpm!). That can be achieved with either a quicker twist or a higher velocity - or both. Twist rates are fixed within a given barrel, so that allows us to adjust velocity. But heavy bullets can't be driven as fast.
In a given barrel, therefore, if a given bullet weight can't be stabilized even at its top speed, the best recourse is to drop to a lighter (or shorter) bullet. Just changing from a super-pointy bullet to a blunter one of the same weight can be enough, all else being equal.0 -
47studebaker,
Generally speaking twist is dependant upon bullet length, which is usually dictated by weight. But velocity plays a minor role in this as well. You can have situations where a twist will stabilize a bullet only if you push it fast enough. I've had a number of those situations. But, unless it is right on the verge of being stable or not at given velocities, I wouldn't try to push too hard to get it to stabilize. Brass trashing and barrel wear aren't worth the hassle vs. the simplicity of going to a shorter (lighter) bullet. You even get some of the wind drift back via gained velocity with lighter bullets.
You should be stabilizing up to 160 gr. bullets with that 1-11" twist. I know I can stabilize up to 154 gr. bullets with my 1-12" twist. Your TCU doesn't push bullets out as fast as my 7x57 hunter though, so maybe only 150/154's. What I load is a 140 gr. bullet with as much 4895 or AA2015 loaded in the case as I can. When I was a kid I got to load this when it was just the 7mm Ugalde. I never owned one until several years back and I wanted one while I could still pick Wes's brain as to what worked or not. He made it simple as pie, 4895 and 140's. I had the good fortune of he and my dad going to school together, being in the NG together and me growing up where he had the opportunity to dispense advice.
Edit:
My 7mm TCU is a 14" barrel with a 1-10" twist and it will stabilize 168 SMK's. pushed @ 1600.
Also, something of note is that with the TC contenders, I think, you get more bullet movement if the pistol is not held firm when fired. I watched the groups one day when I was letting the pistol have too much free recoil when I fired and the groups were terrible. I made a point to control the pistol through the recoil and the groups improved.0 -
switched to Hornady 120's with 24.2 of IMR 3031 group went from a "string 4 inches" long at 100 yards to 3/4" at 100 yards. First loadings were traveling around 2200 FPS. Will check the FPS on this load soon. 0
Please sign in to leave a comment.
Comments
4 comments