1:7 twist 300blk

[cs86]

I tried a few NOE 196gr boolits in my AR 300blkout yesterday testing for functionality. The AR has an AAC 16" barrel with a 1:7 twist button rifling with a pistol gas system. The boolits are made from WW alloy + a little tin all GCed. Powder coated with smokes green powder using the ASBB method one time at 400F for 25min.

I'm concluding that this barrel can't handle pushing lead fast, or it can't push softer lead very fast, or I need a thicker coating. I've tried 150gr, 180gr and now close to 200gr boolits with PC and noticed a few things. When pushing 150gr and 180gr boolits down the barrel it looked like it would lead from about the middle of the barrel to the end and continue to build up. Yesterday I noticed the 200gr boolits would lightly streak lead from about 3-4 inches from the end of the barrel. It appeared to be on the driving groove. I didn't shoot enough 200gr yesterday to see if lead continued to build. I was working with a few bullets at 1gr increments with the powders of lil gun, H110, and IMR 5744 to see where the bolt would lock back. According to the Hornady book I was probably shooting a max speed of 1200 to 1300fps.

It seems like my threshold of speed is going to be about 1000fps for this alloy or coating method. I just received a new mold NOE 147gr and I'm curious to see how it works out. I wish I had a slower twist. I'd like to run suppers at 1800fps, but I'm having my doubts it will happen with this barrel.

Just some results and conclusions of my own that I thought I'd share.

[Jupiter7]

What'd ya size them to? Sounds like you didn't have a good bore fit. Subs run in the 35k psi range, not low pressure at all. PC is only as good as your alloy and bore fit.

[ipijohn]

I run range scrap 312-155's powder coated at 1950 FPS all the time in my 1:8 twist and have NEVER seen any sign of leading.

[cs86]

What'd ya size them to? Sounds like you didn't have a good bore fit. Subs run in the 35k psi range, not low pressure at all. PC is only as good as your alloy and bore fit.

I'm pretty sure I have a good bore fit. I'd have to look back in my records to see what I slugged at, but if I'm remembering right I think the bore slugged at .309 or under. I remembered all the dimensions seemed to be really tight. The throat was right at or a smidge over .311. So I've tried sizing at both .310 and .311. If I wasn't getting a good fit I figured that I'd be getting leading right past the throat which didn't look like I had any trouble with.

[cs86]

I run range scrap 312-155's powder coated at 1950 FPS all the time in my 1:8 twist and have NEVER seen any sign of leading.

I've heard of it done before. I'm not sure how or why I can't get it to work. I've asked the questions before and can't seem to come up with an answer. Wish I had a chronograph to see what I'm getting for speed. I'm just going by the book.

[runfiverun]

I can't even chamber 311 in my AAC bolt gun.
310 will scuff the front drive band and help align things.
but I have to be careful of how much drive band I have in front of the case or it locks things up when chambering.

the barrel will handle fast I have no problem going over 1400 with cast in my 1-7 twist, but the bigger plain based boolits are not built for fast they are built for slow.
plain based boolits have a velocity window in even a 1-12 twist rifle they just don't do well much over 1100 fps.

[Smoke4320]

something is way wrong here ...
Are you curing the PC Powder for 15 Minutes at an ACTUAL 400 degrees ?
I am shooting 247 grain bullets 50/50/2 alloy, PC'ed jet black (DT method 1 coat) at 1425 FPS and getting no leading and great accuracy
same with Mi hec 312 159's 50/50/2 alloy, PC'ed jet black (DT method 1 coat)at 1600 FPS
My bore measures 308 and I am sizing to 311

Please let us help you work thru this .. There is a reason for the leading we just have to get to the bottom of it..
I am doing inventory today and tomorrow so I will not be on here much till Wed but will visit when I can

[cs86]

I can't even chamber 311 in my AAC bolt gun.

It seems like I had trouble with sizing to .311 and getting some to chamber also before yesterdays session. With the test rounds yesterday they had been sized to .309 prior to coating. Then sized to .311 because I already had it set up. I only have a .309 push through. The .310 and .311 are used in a LAMII lube sizer. I chambered a few rounds with .311 sized boolits into the gun prior to loading all of them up and they chambered fine so I went with it. There was enough drive band to run into the throat.

Are you curing the PC Powder for 15 Minutes at an ACTUAL 400 degrees ?

I remember when I first started PCing and the dial on the oven that read 400F was used. The result overbaked some PC that I got through another vendor. It chipped off the boolit during a smash test. So I tested the oven by putting my lead thermometer in the oven and found that I need to set the oven dial to about 350 to reach 400. It seemed to fix my over baking problem. I'm not sure that my lead thermometer is 100% accurate, but it seemed to work with all my pistol rounds so I left it. Maybe I'm to low on temp...? Maybe I can find another thermometer and test the temp again.

What does undercooked PC resemble?

[popper]

I get 1700+ with a PB 150gr using isocore in the 1:7, accuracy is ~2" @ 100, IMR4227. HiTek or Smokes black powder. No leading at all.

[Smoke4320]

Undercooking will not cure the PC completely and give you the exact problem you are describing ..
This MAY NOT be your issue but undercooking WILL give you that result

[cs86]

Smoke, at 400F roughly how long after 15min. can you cook a boolit before it overcooks, or will it over cook if you are truly at 400F? Probably depends on the amount of boolits going in...

I'm going to try and run some oven tests in the next few nights to see if I can get another reading that might be more accurate.

[popper]

I've cooked Smoke's black @ 415 for 1 hour, still works great but size after cooking as the powder seems to shrink during the cooking, .

[Smoke4320]

I have run up the 20 minutes with no ill effects
Never tried any longer
If you oven is at an actual 400 degrees and no hot spots ( why we recommend a convection oven) you will be fine

[gpidaho]

I ran some 230gr. Lee boolits out of my 1 in 7 Handi yesterday they were powder coated and over 16gr. RL7 chronograph said 1523 av without the check and 1540 when checked. No leading 150gr Lee FP lubed over 17.5gr RL7 ran 1758fps av no leading there either. I know that's apples to oranges but as Smoke said something must be wrong in the cure. I don't think it's the 7 twist. Gp

[Smoke4320]

It's not the twist rate ..we have many people shooting 1:7 twists without any leading at all
It's either the cure or a fit issue almost surely

[cs86]

It's either the cure or a fit issue almost surely

I'm pretty sure the fit is good as I did a few slugs and chamber casts before I put the barrel on the gun. I think there could be something off with the cure temp. I wanted to test my oven last night, but had other things come up and forgot about it.

[sinker402]

I'm pretty sure the fit is good as I did a few slugs and chamber casts before I put the barrel on the gun. I think there could be something off with the cure temp. I wanted to test my oven last night, but had other things come up and forgot about it.

Run a cotton ball down the barrel and check around the gas port hole. May need to clean it and then run some j word. Before going to cast again. This fixed my problems

[popper]

COWW or SOWW? SOWW are pretty soft. IIRC when I first started doing ESPC I had a 40SW that would do the same thing if sized too large - cuts on the bore edge and dumped lead 1/2 way down.

[Smoke4320]

Run a cotton ball down the barrel and check around the gas port hole. May need to clean it and then run some j word. Before going to cast again. This fixed my problems

That is a good point as well ..

have seen a few barrels where there was a burr and it took 100-200 bullets jacketed bullets to wear the burr away ..
20-30 jacketed bullets then check with Sweets 7.62 .. if the barrel is rough or the burr is there you will get a pretty blue colored patch .. clean till blue goes away ...OIL barrel to kill any leftover sweets ....shoot 20-30 more and repeat cleaning
do till you see little or no blue
I stay away from using flitz or similar paste to polish the barrel.... Its a personal thing

[Zaneiel]

I run wheel weights with a little tin through my .300 Blackout 1:7 twist 16" so far no leading. I powder coat (it has its own set of issues but that's mostly me being new to powder coating).
I have feed issues with anything 310 and over as well. Or sometimes if there is a little powder coat buildup I will have to fiddle with getting it ejected because it didn't go into battery. I have been using the 230 grain lee boolits with Accurate 1680 powder with good results. I just got the 155 grain gas check mold last week and cast a few but haven't tried them yet. Good luck