300 blackout barrel issue

[John McCorkle]

Thank you guys for compiling such a great site. I've spent some time looking around and have been a passive "learner/reader" for some time now and have spent the past few years casting for pistol. Loving it by the way.

Now I've come to a problem that I haven't been able to find anything for.

So I am most of the way through a 300 blackout pistol build. It's my first 300 build and I have to say I am very excited about the potential for the round. What is most exciting is not only what it can do out of an 8 inch barrel (what I have) but how inexpensive it will be to reload and even cast my own bullets for.

This is where my question comes from though. when I got my new barrel from Santa this year, I slugged the bore to check diameter so I know what to size the cast bullets to. I know from at least some experience that fit is king when it comes to casting...BHN, velocity, gas check, ect...they all play a part but fit is most important. The barrel slugs at .305...and that kinda concerns me. I bought the barrel from MAS defense (or Santa did I should say). I know cast boolits should be around .001 to .002 above bore size to allow perfect fit. what now? This seems like it could be for one out of spec, but I don't know...has anyone else had this experience? Will shooting jacketed bullets be safe through this one? (my understanding is most spec 30 cal bores run .307-.3075 range.)

will sizing to .308 be sufficient for a good solid alloy of wheel weights and a bit of tin/antimony added to sweeten, water dropped, heat treated, and gas checked (which is what I have going on by the by) I have a .309 sizing die but that is .004 over bore size...seems a bit much but want to see what you guys think

thoughts?

I am certain there are may factors that go into play so please forgive my ignorance if I have overlooked anything, just trying to learn

[leadman]

That is an unusual size for a 30 caliber barrel. Is this an odd number of grooves so maybe you are not getting a true reading? Once you determine if the diameter of the grooves actually is then you can proceed.
The alloy will go hand and hand with the weight of the boolit and how fast you want to push it. I shoot air cooled clip on wheel weight alloy gas checked in my 300 Whisper Contender carbine with no problem. The boolit weighs 200 grs. and velocity is usually under 1,100 fps.
If you want to shoot a 120gr boolit with GC at 2,000 fps the alloy will need to be much harder. I find around 17 BHN works well. These velocities are in my 18" barrel so will be much slower for your 8" barrel.
There is alot of good info in Richard Lee's second manual to help you determine pressure and alloy strength. Well worth the money.

[John McCorkle]

Thanks for the guidance, I checked again this morning, slugs rotate (snugly) between the jaws of the caliper at .3055 checked the calibration of the calipers by measuring the sized boolits I have run through the sizing die (spot on at .309)

I used 00 buckshot for the slugs, so i'm headed to the hardware store to grab a few lead fishing sinkers that measure out about the right diameter for slugging to see if I can get a better read or maybe a better fill out in the grooves if that is possibly making the difference.

The boolit mold I have is a lee 160 grain round nose, it's not the cruise missile 230 grain designed for subsonic they just came out with but it's all I need, hoping to push it around 1600 with h110 but that may require a bit of adjustment out of a 8 inch barrel, i'll just have to play around with it and see. Baby steps. This project has been a long while coming and I fight the temptation (and the fear of sleeping on the couch) from buying all the supplies and trinkets I need all at once.

I'll let you know what I find out leadman, oh and yes you are right, it's odd number of grooves. I did notice that before and made sure to get opposite of them to get the read on, but good eye (0=

again, thanks for the effort you guys put into the sport, hobby, and the site...it's been a treasure trove

[BK7saum]

You might try wrapping a thin feeler gauge around the slug and altake a measurement. Then subtract double the gauge width. Measuring odd number groove barrels with a caliper only isn't usually accurate. I bet the groove measurement is much closer to 0.308

[John McCorkle]

that's a good idea, headed to harbor freight now...maybe I can pick up one of those 4" cutoff saws everyone raves about for cutting 223 brass to make 300 blk

[John McCorkle]

Got the feeler guage, and i'll be dog gone if you weren't right, just a tad under .308. Got the cutoff saw while at Harbor Freight too...seems to be working like a charm,

Thanks so much for the input. I'll keep you posted on how the build and more importantly the load up starts to look.