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

[OnyxSkyDV]

So, having a little issue hopefully you guys can give me some suggestions on.

Using an MP-Mold 311-410 I am casting and powder coating HP bullets for my 300blk. Sized to .309, and have tried with and without PB gas checks.

While some of them seem to work just fine, it seems that occasionally I will end up getting a small ring of PC & Lead that builds up between the mouth of the brass and the lip that sits right before the throat of the chamber.

My brass is getting trimmed to the right length, so I know it's not running short, but once that ring gets left in the chamber I have to pull a boresnake through from the muzzle to dislodge it before I can get the rounds to chamber and seat fully.

I chambered a fired round and took a photo with the borescope to show the gap were I am getting the build up.

270621

Any suggestions on how to prevent that build up from happening? Short of trying to trying to machine out that lip and make it more ramp like (not sure if it's needed or not) I don't know how to prevent this. I'm worried that if I sized the bullet down anymore I would get some gas cutting up the sides.

Thanks for any input!

[Misery-Whip]

Can you give us a pic with it clean?

[John McCorkle]

Brass sizing length. When I first started casting for 300 blk I had the same issue. The brass needs to be sized longer for your chamber by some amount.. I can't tell you exactly what that would be but.

What is happening is the pressure behind that boolit is so much that the boolit is obturating in the bore. It's finding a gap between the case mouth and the throat that shaves off a ring as the boolit moves forward during firing.

You'll find that ring is the exact thickness of gap between the length of your sized brass and the throat

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[John McCorkle]

Oh and you'll also find that you'll get leading as there is now an exposed ring on the boolit that is being driven across that steel

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[OnyxSkyDV]

I will have to try forming some more brass and figure out the length I would need to close that gap. Great idea though, I will have to see what I can do with that.

Thanks!

[SweetMk]

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,, :mrgreen:

[gpidaho]

This is just a thought with no science backing it but you might want to get one of those chamber buttons and measure the true length of your rifles throat. If the brass is trimmed short of the true length of the throat by quite a bit and the transition is abrupt where the throat meets the rifling it could be that the boolit is swaging up in the throat before it reaches the rifling and being cut by an abrupt edge. A lot of barrels are built with firing jacketed bullets in mind. Gp

[yovinny]

So if Im understanding correctly,, the black part is the actual front of the chamber and the color change difference right to the left of it is where his fired case actually ended ?

[OnyxSkyDV]

I tried to label it, but let me see if I can describe the pic, I can understand why it's tough to visualize.

I have a sideways viewing bore scope inserted into the muzzle, and it's all the way down to a piece of fired brass that is chambered in the rifle. (The left side is the brass) We are viewing this from literally inside the neck of the empty brass.

On the right side is the throat of the chamber. You can see there is a gap between where the brass ends and the throat starts. (Labeled in black)

It sounds as though I need to trim my brass longer than spec to get it to close that gap so I am not shaving the bullet as it goes by.

Hopefully that helps some.

[John McCorkle]

Chamber casting with a piece of brass you know currently is too short would also give you a dead nutz picture of the front of that throat too. You'd be able to measure the amount of lead/casting alloy in front of the case mouth on a piece of brass you previously measured the length on..

You can use chamber cast alloy or pound a dead soft piece of lead in the old school way....either way would give you a perfect example of what you are looking for.

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[SweetMk]

On the right side is the throat of the chamber. You can see there is a gap between where the brass ends and the throat starts. (Labeled in black)

It sounds as though I need to trim my brass longer than spec to get it to close that gap so I am not shaving the bullet as it goes by.
.

I think what is happening happens quite often in the rimfire revolvers that I own,,

If I only shoot 22LR in the revolver, it shoots and ejects, perfectly.
BUT, if I fire some 22Shorts in the revolver, THEN go back to 22LR, the brass will stick in the gun.

The 22LR will hang up on the ring of some deposit left by the 22Short, right at the end of the brass.

Likewise, if I shoot "Stingers" after shooting 22LR, or 22Shorts,, the Stingers longer brass will hang on eithers deposit.

I guess what I am saying is this may be perfectly normal,, it is gonna be tough to have brass long enough to reach the rifling,,
and, not cause chambering issues,, there has to be a tolerance,,, that is more than a few thousandths of an inch,,, IMHO,,

[mattw]

When I ran into this problem, I had to seat just a bit deeper. My guess is that you have just a bit of the fill size driving band in front of the case mouth, when shooting it is being shaved a bit and creating the ring. I ran into this problem and it was frustrating.

[yovinny]

I tried to label it, but let me see if I can describe the pic, I can understand why it's tough to visualize.

I have a sideways viewing bore scope inserted into the muzzle, and it's all the way down to a piece of fired brass that is chambered in the rifle. (The left side is the brass) We are viewing this from literally inside the neck of the empty brass.

On the right side is the throat of the chamber. You can see there is a gap between where the brass ends and the throat starts. (Labeled in black)

It sounds as though I need to trim my brass longer than spec to get it to close that gap so I am not shaving the bullet as it goes by.

Hopefully that helps some.

Thanks,, that makes much more sense now and I can see how whats been said applies.
Im just surprised the actual throat seems so short.. I thought BO's were supost to be throated very long for those long, heavy subsonic bullets..?

[OnyxSkyDV]

I may be mistaken, but I thought the throat was the part where the rifling starts, and it was how far back and how shallow the rifling is before it gets to the full thickness of the lands.

This I think is the chamber itself is longer than the spec calls for. Makes no issue with FMJ rounds, but clearly causes some issues for me with cast.

[OnyxSkyDV]

When I ran into this problem, I had to seat just a bit deeper. My guess is that you have just a bit of the fill size driving band in front of the case mouth, when shooting it is being shaved a bit and creating the ring. I ran into this problem and it was frustrating.

I'm worried that even seating the bullet deeper, I still have that gap with a sharp edge on the lip of it that will still shave some of the bullet on the way by. I think I need a decent chamber cast to figure out whats going on for sure there.

[John McCorkle]

I will say having that bore scope sure helps....which one is that?

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[mehavey]

...occasionally I will end up getting a small ring of PC & Lead that builds
up between the mouth of the brass and the lip that sits right before the
throat of the chamber.Are you sure the PC is absolutely fully cured, passes the "smash" test,
and cannot be scraped from the bullet on its own ?

ps: what powder and what process are you using ?

[OnyxSkyDV]

Are you sure the PC is absolutely fully cured, passes the "smash" test,
and cannot be scraped from the bullet on its own ?

ps: what powder and what process are you using ?

Passed the smash test just great, been curing for 2 weeks maybe. ES applied powder from HF gun.

[mehavey]

What/whose powder ?

postscript: Properly heat cured (Eastwood) powders are totally cured at
the end of 400degr/20-min cycle. No further (effective) curing occurs.
(They say... :popcorn:)



400degr item, not just oven temp.

[OnyxSkyDV]

I will say having that bore scope sure helps....which one is that?

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I'd been wanting a borescope for a while, but they were always more than I wanted to spend on one. Then I saw one recommended from Amazon, its .2" diameter, so won't work for my 17cals, but should for everything else. Its USB, you plug it into your computer and just run the Camera program if you have Win10 and its right there, larger than life.

For the price I am amazed at how good it is. They have a cheaper version with a flexible shaft, but I felt the solid shaft was a better choice for me.

Hopefully its ok to post the link, I am not affiliated with them in any way, other than being happy with their product.
https://smile.amazon.com/Teslong-Borescope-Side-View-Windows-MacBook/dp/B081T1DN1P/ref=sr_1_6?dchild=1&keywords=teslong+borescope&qid=1604367063&sr=8-6

[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?? :D

(NOTE, the BUT joke above WAS intended,,, LOL!) [smilie=1:

[OnyxSkyDV]

BUT,, is this one proctologist approved?? :D

(NOTE, the BUT joke above WAS intended,,, LOL!) [smilie=1:

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.
270648

[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
271227

[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,, :mrgreen:

I used the Precision Cast Bullet Design ~ Ultimate (https://www.tmtpages.com/#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.


http://www.tmtpages.com/LinkSkyImages/forum_images/300%20Blackout/300_AAC_Blackout_SAAMI_Rifle~Barrel~500-pi_Lg_300_AAC_Blackout_SAAMI_Rifle~500-pi_A_TMT_Design_309-300-190~500-pi_Lg.png


http://www.tmtpages.com/LinkSkyImages/forum_images/300%20Blackout/300_AAC_Blackout_SAAMI_TMT_Design_309-300-190_Lg.png

[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.