Question about throating reamer

[mroliver77]

I have a "target" rifle a friend built for me out of an Type 38 Arisaka action. It seemed like a good Idea at the time :) I bought a Shilen 10 twist .30 cal blank and he fitted it full size to my action. He fit an aluminum block to the barrel just in front of the action and bedded this in a rem 40 x bench stock that was channeled and fit with an aluminum strengthening block.. The rest of the barrel and the action are completely floated. It also is fitted with a Timney featherweight trigger for a mauser. Scope is cantilever mounted to the bedding block. Looking to shoot only cast I chambered it in 7.62 X 39 as this seemed like a decent easily had case and I have a nice supply of REm and Lapua brass. We never got aroungd to Lenthening the throat to fit my boolit. I have a 150 LBT and a 170 lbt both are full diameter boolit designs. I cannot find much information on throater specs. If I have to have a custom made I am not sure what would be the most cast friendly design. Would I want to send the reamer guy a boolit and let him decide? I got gun out and messed with it some yesterday and man does that case have a short neck! I have plenty of room to rechamber to something else but I am a tightwad and do not want to go with too much of a custom sesign. I liked The idea of using a short chambered .30-06. and cutting down dies to match. Any sugestions, comments about this? I know I messed up with the 10 twist but I will live with this.
J

[felix]

Jay, you are presenting a tough situation. My personal objective would be to make the thing shoot 2400 fps (typical lead limit) with the boolit weight of choice while using current powder formulations. So, that would mean to change boolits in your situation or increase the size of the case. The actual speed of shooting would be limited by the RPM potential of the boolit design, and the powder speed used to give at least 35K CUP at that boolit speed. You need to think about this for a while. I have had Forester make custom dies for my guns, and that seems to be the way to go for your situation no matter what case size you really want. BR hand dies are too good for your application and their potential won't be realized. Hand dies are far to slow in use anyway. ... felix

[danski26]

I'm not sure that you have to lengthen the throat. As long as you can get the velocity you want with the case capacity you have "with the boolit pushed back into the case" you should be fine. As you shoot your throat will naturally lenghten and this is usually the destroyer of accuracy in a barrel. When the throat gets too long to seat the boolit at the optimum distance from the lands.

[WKAYE]

mroliver77,
Go to Pacific Precision Grinding's site and have a look at their uni-throater. I have one in 308 and another in 223. They're the cat's meow for throating for a specific bullet. :Fire:

[mroliver77]

I'm not sure that you have to lengthen the throat. As long as you can get the velocity you want with the case capacity you have "with the boolit pushed back into the case" you should be fine. As you shoot your throat will naturally lenghten and this is usually the destroyer of accuracy in a barrel. When the throat gets too long to seat the boolit at the optimum distance from the lands.

It would put 90% of the boolit in the case! lol I need all the boiler room I can get to push this boolit.
I dont think it is possible to get the 2400fps as Felix sugests but 2000 ought to be very doable with 100% loading density. I see in the CBA records that lots of shooters run their boolits at 1900 -2400 with most being on the slower end of this.
I originally wanted to go with the .308X1.5 but custom everthing just wasnt in the budget. With the 30 BR being a pretty regular round these days I might rechamber to it if the $$ situation ever improves. For right now if I throat to seat boolits out where gascheck is flush with neck origin it should work OK.
WKAYE, thanks for the info.
J

[felix]

Yeah, Jay, as long as you are below say 150K RPM you should be in the ball park of a reasonable load. I think several of us demonstrated that is prolly a max demarcation for a 30 caliber boolit. Naturally, this RPM business is a function of the diameter and quality of the projectile, because we are talking angular inertia, i.e., the amount of rotational action to begin to pull off mass from the center line. 1800 fps will be 130K for a 10 twist. ... felix

[mroliver77]

Felix,
Is alloy a factor in this. I mean will lino or No2ish (antimony=tin)alloy help or hinder this effect?
2000fps being 144000rpm,
J

[runfiverun]

i think i would go with 2-ish to start with and shoot for about 1800 fps
check for leading
and play with the load for accuracy
an accurate load is better than velocity any day

runfiverun

[felix]

Yes, Jay, alloy is always a factor. However, it has more value in internal ballistics rather than external. In other words, lino should grab a faster twist better, giving more consistent shot-to-shot acceleration. In practice, however, this might be bunk because different powder ignition parameters play a significant accuracy role, and soft boolits can really shine at the target. Condoms (jacketed) bullets do a better job in holding together at RPM because the copper acts as a straight jacket. Again, condoms might not fit perfectly either, like a terribly hard boolit, immediately after ignition. ... felix

[Johnw...ski]

Yes, Jay, alloy is always a factor. However, it has more value in internal ballistics rather than external. In other words, lino should grab a faster twist better, giving more consistent shot-to-shot acceleration. In practice, however, this might be bunk because different powder ignition parameters play a significant accuracy role, and soft boolits can really shine at the target. Condoms (jacketed) bullets do a better job in holding together at RPM because the copper acts as a straight jacket. Again, condoms might not fit perfectly either, like a terribly hard boolit, immediately after ignition. ... felix

Hi Felix,
I have a 45-70 I am shooting and a 35 Whelen I intend to shoot in the near future, I will be shooting both with gas checked bullets.
They both have a 1-14" twist
What rpm limits should I be looking at?
Do gas checks vs. PB affect the rpm limits?
This rpm's of the bullet is new to me, where can I read up on it?

Thanks,

John

[felix]

John, the best place to read about these RPM functional descriptions is right here on this web site. RPM is another measure of a boolit's potential performance, just like the ballistic coefficient is. RPM is a function of the barrel twist rate. The harder/tougher the boolit is, the more RPM can be delivered to it without internal boolit destruction; the barrel walls will hold the boolit. The gas check helps in this endeavor as well, and most especially at the base of the boolit before the boolit gets expanded too much in that area. The boolit's accuracy will depend on the balance of the boolit outside of the barrel. Cast boolits are more suspect about balance when contrasted to swaged boolits/bullets because of random air holes within the boolits. It is always smart to not overspin a projectile because no projectile is perfect in balance, and most especially after being thrown out of a gun at high speed. Add to the problems with those given by the muzzle crown. Spend time searching around this board for some heated discussions about the matter. It seems to me that all posts are correct when various "facts about a boolit's life" are taken into account. ... felix