Bore diameter question - chamber cast, slugging, etc.

[Ggmac]

Ummm my understanding is that the cast is supposed to shrink initially (first couple of minutes) then start expanding. At one hour it should have been the same size as the chamber; by now it should be *bigger* than the original dimension. Since it *now* only barely measures the bullet diameter...

...though I agree with the other observations that nothing else on the primer or case head seems to indicate over pressure.

Yes , what I'm trying to determine is if you cast is correct . By measuring your fired case neck and comparing it to the cast you can verify the cast numbers are either correct or off . I hope I'm explaining correctly .

 

[Ggmac]

No

What I should explain is I've used cerrosafe for decades and it has shrunk as expected and other times has not . I've always explained it ( best I can determine ) either casting temp , age , # of times I reused it ( contaminates?) or different ingredients for different batches of cerrosafe .

 

[geo.ulrich]

Well you do not have excessive pressure, that's good. A .073 firing pin hole is real big especially with an .063 pin diameter . 068 is better but still pretty bad. Protrusion at .062 is high but you fixed that. I know you said you set headspace on barrel but have you checked to a fired case and an unfired round. If your gage is to the long side and factory loads are to the minimum you could still have a headspace issue. Free bore on a factory barrel will be up to .002 larger in diameter you should be able to borescope and see if reamer cut all the way to the groove dia. if it didn't you will see the edges of grooves did not clean up.

 

[butchlambert]

I'm relatively new to pin gages... started using them on case necks last year, finally got tired of ordering one or two at a time and bought two sets, from 0.011 to 0.250". It never occurred to me before to try one in the neck... Kinda worried I'd get it stuck, knowing my luck

Monte, sounds like you have pins in .001 increments not .0001. Shouldn't have any problem getting it stuck in the bore.

 

[memilanuk]

What I should explain is I've used cerrosafe for decades and it has shrunk as expected and other times has not . I've always explained it ( best I can determine ) either casting temp , age , # of times I reused it ( contaminates?) or different ingredients for different batches of cerrosafe .

Huh. And here I was being assured that it was this wonder material that *always* expanded to *exactly* the right dimensions, that tool & die makers swore by the stuff, yada yada yada.

FWIW, the fired case necks measure ~0.253, and the cast neck measures ~0.2535-0.254", depending where/how I get on it (was kind of in a hurry).

 

[memilanuk]

Well you do not have excessive pressure, that's good. A .073 firing pin hole is real big especially with an .063 pin diameter . 068 is better but still pretty bad. Protrusion at .062 is high but you fixed that.

Maybe I've been living in .308-land for too long... while I generally prefer a tighter fit, and some of my guns do have bushed firing pin holes... I wouldn't really expect this kind of problem from a .223 Rem. Something like a 6 BR or 6-6.5x47L, sure, I've had issues with those little monsters *needing* bushed firing pins to function normally without cratering.

I know you said you set headspace on barrel but have you checked to a fired case and an unfired round. If your gage is to the long side and factory loads are to the minimum you could still have a headspace issue.

So when I went back and re-checked the headspace, I made sure that it would close (with a completely stripped bolt) freely on a Forster GO gauge. Then I verified that it wouldn't close on a NO-GO gauge. Then, just to check, I took a loaded round (again, stripped bolt) and verified that the bolt dropped freely closed on *that* too. Not even a whisker of resistance, so I'm reasonably confident that it isn't contacting places that it shouldn't.

Free bore on a factory barrel will be up to .002 larger in diameter you should be able to borescope and see if reamer cut all the way to the groove dia. if it didn't you will see the edges of grooves did not clean up.

Yes. Like I said, nothing jumped out at me when I scoped it. And the chamber cast does not show the rifling extending back to the case mouth or any of the other weird stuff I've ran into before.

 

[memilanuk]

FWIW, I have a bolt on its way back from @Grimstod with bushed heads (.308/.223) and matching firing pin for another gun (10 BA Stealth). Ultimately *that* is the gun this barrel is intended for. Hopefully all the problems will 'go away' with a properly bushed firing pin. At least the symptoms will; I just want to be sure that there isn't something *else* lurking that I'm missing. I though an unusually tight freebore might be something to be concerned about. Apparently even the vaunted Cerrosafe isn't 100%...

 

[Dusty Stevens]

FWIW, I have a bolt on its way back from @Grimstod with bushed heads (.308/.223) and matching firing pin for another gun (10 BA Stealth). Ultimately *that* is the gun this barrel is intended for. Hopefully all the problems will 'go away' with a properly bushed firing pin. At least the symptoms will; I just want to be sure that there isn't something *else* lurking that I'm missing. I though an unusually tight freebore might be something to be concerned about. Apparently even the vaunted Cerrosafe isn't 100%...

Keep us informed on your results please. Too many times if a problem is solved we never hear about the particulars

 

[carlsbad]

This is way more complicated than it should be. First, expansion is in in/in so a 1" diameter round would expand by .0025. A .250 diameter casting will expand by 1/4 that or .00063. So your neck isn't as tight as you think.

Doesn't sound to me like there is anything in your chamber that would cause high pressure. Bush your firing pin and you'll never have another problem. Grimstod or I can bush it and have it back to you in less than a week.

--Jerry

 

[memilanuk]

This is way more complicated than it should be. First, expansion is in in/in so a 1" diameter round would expand by .0025. A .250 diameter casting will expand by 1/4 that or .00063. So your neck isn't as tight as you think.

Good to know. I wish the info from Brownell's was that specific. When it said 0.0025" (over 200 hrs) my question was whether that was 'total' i.e. .226 + .0025 = .2285, or each side i.e. .226 + .0025 + .0025 = .2310". Still, I'm a little surprised its *smaller* than expected... I would have expected at least .225 or .226 dia. FB on a SAAMI chamber - not less than .224". But the 'relative' nature of the expansion makes a *lot* more sense than what I was being told (basically they quoted the info from the Brownell's catalog page to me). The lot-to-lot variability kind of concerns me a fuzz, though.

 

[carlsbad]

So you take your final measurement and subtract the expansion. So if you measure the freebore area as .224 and the expansion is .0006, then your final diameter is .2234. I don't know what you are measuring with but I wouldn't expect better than =/- .001" accuracy unless you're an experienced machinist or equivalent. The bore of most barrels is .224 so you're right there where you should be. I wouldn't expect the freebore to be much greater than the barrel ID. I just checked the drawing for my 223 issf reamer and the fb area is .2242. --Jerry

 

[memilanuk]

Well... hmm. Interesting

I was going to say that yes, in my experience with .308 Win, most of the 'match' chambers I'm familiar with run a .3085" dia. freebore - but SAAMI freebore is .310". Hence, I'd expect the SAAMI freebore for .223 Rem to be something similar - .224 (bullet/groove diameter) + .002, or about .226".

Funny thing is, looking at the online SAAMI drawings... it sure *looks* like it's calling out the (very short) freebore diameter as ' .224" '... Just because that seems awful odd I checked the drawings for .222 Rem and .222 Rem Mag, just because of the close relationship... and those both show ' .224" ' for the freebore diameter as well.

I woulda never thought that. So apparently the case neck @ .253-ish, and the freebore @ .2235-.224-ish are pretty much right where they should be...

I definitely learned something new today!

 

[Grimstod]

FWIW, I have a bolt on its way back from @Grimstod with bushed heads (.308/.223) and matching firing pin for another gun (10 BA Stealth). Ultimately *that* is the gun this barrel is intended for. Hopefully all the problems will 'go away' with a properly bushed firing pin. At least the symptoms will; I just want to be sure that there isn't something *else* lurking that I'm missing. I though an unusually tight freebore might be something to be concerned about. Apparently even the vaunted Cerrosafe isn't 100%...

I can pretty much guarantee the the bushing will solve your problem. I looked at your photo and it is because of an oversized firing pin hole that your are getting cratering and posed primers. In the future now that your bolt is bushed always judge pressure by how flat the edges of the primer are and case head expansion. Since you will never see cratering or rupturing again those are the two methods you will have to use to judge pressure. Also please do keep us posted on your progress.

 

[memilanuk]

Any thoughts as to why it showed so much worse with some ammo than others? Just the difference in primers?

 

[geo.ulrich]

Any thoughts as to why it showed so much worse with some ammo than others? Just the difference in primers?

most probably...cup thickness and hardness

 

[Grimstod]

most probably...cup thickness and hardness

That is what i was going to say. Also variations in the amount of powder. Factory stuff is not that consistent.

 

[memilanuk]

@Grimstod I was referring more to that it cratered badly and pierced on the Hornady 73 ELD-M / 75 BTHP ammo (with crimped in primers), cratered less so on the Federal S77MK ammo, and not at all on the 55 FMJ bulk ball ammo.

 

[rover31]

most probably...cup thickness and hardness

Pretty sure that’s what I said...