Improved BBT ?

[sb327]

Got the mold cut. I still have a little adjusting to do on the pin length.

Then maybe get a few made up.

Thanks for the research and drawings Bob.

Dave

The last pic is one pushed through the bore of a TJ.

[rsterne]

That looks pretty good, IMO....

Bob

[Greg_E]

I like the adjustable pin. Looks like a good job cutting that out.

[sb327]

Bob,

I have a 17hmr barrel I’m considering putting on my newest pilot valve build. I’m pretty sure it’s a 10 twist. I can put a lot of pressure behind it (up to 4500). I can get a solid 24” out of this blank. What weight/length .172bbt would you suggest if I were to cut a mold?

Dave

[rsterne]

I would think you are going to be twist limited, rather than not have enough power.... Twist rate is proportional to caliber, so the 2.5 cal. long design, in Reply #6 on page 1, would probably be the best choice.... I know it is only 22-23 gr. but anything longer may not stabilize in a 10" twist.... I have a 9" twist barrel in .172 cal, and the longer Varmint-Knocker slugs did not stabilze in that.... 32.7 gr. tumbled, and the 30 gr. (0.559" long) were not accurate....

https://www.gatewaytoairguns.org/GTA/index.php?topic=171084.0

https://www.gatewaytoairguns.org/GTA/index.php?topic=175620.

For my 9" twist I think the maximum length is about 3 calibers (Reply #17, page 1), so for your 10" twist, shorter would likely be necessary.... Get some samples from Dale of various lengths before you make a mold....

Bob

[sb327]

I will get a better twist dimension when I get home to confirm if it is a 10. I may not mess with it unless it turns out to be a 9.

I appreciate the info and the links.

How was accuracy at the higher end of your power range with that setup?

Dave

[rsterne]

I wasn't testing for accuracy, and I did not tune the muzzle weight at all.... The 10-shot group size at 100 yards, and 5 shot at 50 yards are in the last link....

Bob

[sb327]

I see em in the chart now. Seems like accuracy favored the higher velocity.

Thanks Bob,
Dave