Issues with cast & PC'ed bullets in 300Blk AR15.

[OnyxSkyDV]

What/whose powder ?

I believe this one is the PBtP one that I bought, but also happened with the Eastwood powder I tried.

I have a board with nails through it, alum plate underneath to carry the ground, I spray about 50, then put them in the oven at 420 for 30min.

[OnyxSkyDV]

P.S. appreciate the input guys, I am new to casting and coating, so it's entirely possible that I've messed something up along the way, though I think I am pretty close to getting it right.

The 45-70's sure come out nice.

[popper]

Soft alloy will do that. Short neck can cause it, too large dia can also. Alloy fill the empty space and bore edge shears it. Lots of 9mm have the same problem. Try heat treating, dump from oven into ice water, load week or so later.

[SweetMk]

I'd been wanting a borescope for a while,

Then I saw one recommended from Amazon, its .2" diameter,

and its right there, larger than life.

For the price I am amazed at how good it is

BUT,, is this one proctologist approved??

(NOTE, the BUT joke above WAS intended,,, LOL!)

[OnyxSkyDV]

BUT,, is this one proctologist approved??

(NOTE, the BUT joke above WAS intended,,, LOL!)

Not sure if its approved, butt you'd never guess what I found up there!!!!

I will say I was so impressed we got the otoscope version to use for the kids ears. pretty crazy how good the tech is now.

[mehavey]

...put them in the oven at 420 for 30min.

Well, that ought to do it.

At the same time I'm having a hard time figuring out why
this wouldn't be a routine problem with all bottleneck cases
where there's commonly a neck gap.

`tis a puzzlement....

[cstrickland]

Onyx,

are you using factory 300 blk brass or converted ?? If converted can I ask was it by you or someone else ? If by you what dies and shell holder did you use. It really looks like a short neck to me.

I understand the case OAL may be correct, but is the shoulder set back properly ?? I seem to remember reading somewhere that there was an issue with forming brass with a lee die, and a lee shell holder. It would not size full length and the shoulder was not pushed back to proper dimensions. If I recall for most it was enough to keep the bolt from closing all the way so it was noticeable. I could wrong as I am going form memory , and well I am getting old !!! .

I started with lee dies and did not have an issue myself, but then switched to RCBS shortly after when I found them in stock.

[popper]

That chamber is cut wrong! Should have a step at the case mouth, then a straight (slight taper) for ~ .2" then the bore/lands. Look at a chamber drawing. Trimming longer should help. Getting a proper barrel would be better.

[OnyxSkyDV]

Onyx,

are you using factory 300 blk brass or converted ?? If converted can I ask was it by you or someone else ? If by you what dies and shell holder did you use. It really looks like a short neck to me.

I understand the case OAL may be correct, but is the shoulder set back properly ?? I seem to remember reading somewhere that there was an issue with forming brass with a lee die, and a lee shell holder. It would not size full length and the shoulder was not pushed back to proper dimensions. If I recall for most it was enough to keep the bolt from closing all the way so it was noticeable. I could wrong as I am going form memory , and well I am getting old !!! .

I started with lee dies and did not have an issue myself, but then switched to RCBS shortly after when I found them in stock.

They are converted Lake City brass, by me, and I am using Lee dies. Its done in a Dillon 650 press though, so not the Lee base plate.

I trim with a homemade jig for the harbor freight mini chop saw, then I wet tumble and then size. After sizing I use the WFT trimmer to bring it to spec, and that tool index's off the shoulder for its measurement.

Also, post processing I test them in a case gauge, and they are in spec per that, so if the shoulder was not getting pushed back far enoughI would think the base of the brass would not be flush with the gauge like it is. Its possible the brass is out of spec, but I don't think it is based on the tools I have at hand. Well worth checking though.

I just pulled up the saami specs for the round and it appears that I am within spec according to my micrometer.

[OnyxSkyDV]

I am starting to think that chamber is cut a touch long, though that is a Noveske barrel, I would have expected better from them.

[popper]

I posted the chamber spec, that is what you compare to. Your scope pic just doesn't look right.

[Bashby]

Are you trimming brass to the short end of the acceptable range? Seems to me that since you are making your own brass you could get the length just right so you get that gap down to .001-.002.

You may have cost me money... that borescope looks pretty cool.

[mnewcomb59]

You can coat a dome head brass bolt with lapping compound and LIGHTLY break the corner at the beginning of the throat. I have had 9mms with slightly long chambers with a square sharp throat and they would cut a ring of lead or PC, especially with softer alloys that obturate more.

After beveling the edge with a dome head brass bolt and lapping compound the problem is gone.

[Bashby]

Just shot about 40 rounds through my 300 ar and inspected the chamber. I pulled out a ring of lead from the same area OP is looking at. Tried to upload a picture of it but the browser crashes every time I try to upload. I didn’t have any feeding problems, but I suppose if I had a round with longer case length I would have.

ETA: here it is

[Forrest r]

Odd that chamber doesn't appear to have any freebore.

[Tom Myers]

I would really love if someone could make a sketch of that pic,, detailing a little more as to what I am looking at,,, please,,

Is it a bore pic? is an empty brass in the bore?
I imagine that is not a round in the chamber, that would not leave much room for the bore-scope,,

I used the Precision Cast Bullet Design ~ Ultimate software to produce a chamber image of the 300 Blackout SAAMI spec chamber.
SAAMI specs the cartridge at 0.002" shorter than the chamber.

As you can see from the image, the SAAMI chamber specifies a freebore approximately 0.200" in length.
The first image shows the entire chamber, cartridge and a bullet designed to expressly fit the SAAMI 300 Blackout chamber.
The second image is a larger scale showing the shoulder, neck and throat area.

The photo that the bore scope produced shows the area just in front of the cartridge case to the start of the almost non-existant freebore on the OP's chamber.
The black line in the photo is apparently the 0.013" step down from the chamber neck end diameter down to the freebore diameter.
As you can see in the photo, the cartridge case neck is quite a distance from the black line that is the step down to the non-existant freebore.

Hope this helps.

[jt1]

Tom, those are awesome images showing the dimensions. Thanks for posting them.

John

[Bashby]

Just wondering if OP has looked into this any further. I don’t think you ever said if you are making your own brass or not. If you are you could make one a little longer than spec and see if it will chamber. Look at the gap or lack of gap with the borescope, then trim a few thousandths.... repeat until you see a gap start to appear to find a case length that will work for you.

[SweetMk]

If you are you could make one a little longer than spec and see if it will chamber.

Looking at that fabulous drawing,, I do not think I would ever make brass longer,,
If longer brass is incorrect, and the chamber causes the brass to crimp onto the bullet,,,
there may be a situation where the bullet can not leave the gun,,,

I do not know if a gun is strong enough to swage all of the bullet that is behind the immovable crimp?

[Bashby]

Looking at that fabulous drawing,, I do not think I would ever make brass longer,,
If longer brass is incorrect, and the chamber causes the brass to crimp onto the bullet,,,
there may be a situation where the bullet can not leave the gun,,,

I do not know if a gun is strong enough to swage all of the bullet that is behind the immovable crimp?

I hope I was clear that I am not saying to load a cartridge with an extra long case and fire it, I am saying to start with a long case, chamber it with no powder/boolit/primer, then look at it with a borescope. Trim a little at a time until you see a gap start to appear, then you will know the length to trim the brass to so it all but eliminates that gap.