Bore diameter question - chamber cast, slugging, etc.

[memilanuk]

So... I have a 'problem child' barrel that is cratering, and sometimes piercing, primers on factory ammo. It's more pronounced on some brands than others... I've checked most of the usual suspects - headspace, firing pin protrusion, the firing pin tip itself, main springs, etc. Ran a borescope into the chamber and bore, didn't see anything unusual that jumped out at me. Did a seating depth test with a similar style of bullet, and it did seem to have a very short throat.

Finally broke down and ordered some Cerrosafe chamber casting material. Oiled up the chamber, plugged the bore... made a heckuva mess the first attempt but on the second I got what looks like a relatively decent cast of the chamber and throat.

Two things kind of jump out at me: one, the freebore looks longer than what I would have expected from the seating depth test. And two, the freebore diameter seems pretty tight. If I'm reading the destructions from Brownell's correctly:

Cerrosafe shrinks during the first 30 minutes of cooling and then at the end of an hour, is EXACTLY chamber size. At the end of 200 hours it will have expanded approximately .0025".
​

I cast it late yesterday afternoon. Because of timing and other issues, I measured it at approx. 45-50 minutes, and around the 2 hour mark... at a .2225-.2230" diameter for the freebore. This morning, it's a little under .224"...

Now most of my experience with freebore dimensions indicates that they're usually somewhere between a half thou (for match chambers) to 2 thou (SAAMI-spec) over bullet diameter. If the chamber cast is into the 'expansion' phase, and it's *still* barely at bullet diameter... seems like I might have an extremely tight bore.

I can take another cast if needed, and hopefully this time be able to measure it at exactly the one hour mark, where it's supposed to be right at nominal dimensions. Alternately, would something like slugging the bore with a piece of lead shot be a better option? Never done that before, but I'm open to ideas here.

 

[gunsandgunsmithing]

First things first...What are you measuring with? It should be a good micrometer, not calipers.

 

[memilanuk]

Yes.

 

[geo.ulrich]

can you post a picture of fired case with a primer in it not a pierced one just cratered..a little under groove dia. is not going to cause primer failure with factory loads

 

[gunsandgunsmithing]

Also, do you have a small set of pin gauges? Primer cratering is usually narrowed down to too much clearance around the pin or a weak fp spring, usually the prior. I like no more than about .001-.0015" clearance between the pin and the hole. What kind of action is it? Lots of factory actions have too much clearance (Remington)and Savage is bad about the hole being somewhat radiused(sp). Remington has been doing a lot of this of late...more like a chamfer on them. Cratering is usually NOT a pressure sign.

 

[memilanuk]

 

[gunsandgunsmithing]

Does not look like pressure. Let George reply but that looks like a pin/hole clearance or fp spring problem. Spring is cheapest and won't hurt to try but probably needs to have the bolt face bushed. Again, what action, Monte?

 

[memilanuk]

Also, do you have a small set of pin gauges? Primer cratering is usually narrowed down to too much clearance around the pin or a weak fp spring, usually the prior. I like no more than about .001-.0015" clearance between the pin and the hole. What kind of action is it? Lots of factory actions have too much clearance (Remington)and Savage is bad about the hole being somewhat radiused(sp). Remington has been doing a lot of this of late...more like a chamfer on them. Cratering is usually NOT a pressure sign.

Savage action from a 12 LRP. Factory 223 bolt head. Original firing pin was a fuzz small... 62-63 thou at the tip. Swapped it out for a brand new one that measures 68 thou. The hole in the bolt face measures (via pin gage) 73 thou... about typical in my experience. Shouldn't be cratering and piercing with factory ammo.

One more thing... It craters badly, and pierces, on Hornady factory match 73 & 75 gn loads. It craters, but not as bad and doesn't pierce, with Federal 77 gn loads. It doesn't crater at all on bulk 55 FMJ ammo.

We also swapped bolts from another known good gun, also set up and running as a 223, and it still cratered & pierced primers (and yes, we re-headspace the barrel accordingly). We did not try my bolt in the other gun, nor any of the ammo.

 

[memilanuk]

Back to my *original* question... What would you expect the freebore diameter to measure on the chamber cast?

 

[Ggmac]

Doesn't look like a pressure problem . Did you do anything to ease bolt lift ? Change or cut fp spring or adjust its position ?

 

[butchlambert]

Back to my *original* question... What would you expect the freebore diameter to measure on the chamber cast?

Monte, I can let you borrow my Deltronic pin gauges. My 223 set has 25 pins in .0001 increments. I personally don't think that is your problem, but they're yours to use.

 

[Ggmac]

Back to my *original* question... What would you expect the freebore diameter to measure on the chamber cast
Would need to know if a factory barrel or custom . If custom what's stamped on the barrel . 556 is much different than 223 saami

 

[memilanuk]

Doesn't look like a pressure problem . Did you do anything to ease bolt lift ? Change or cut fp spring or adjust its position ?

No. When the first primer pierced, I checked the FP protrusion. It was a fuzz on the long side (63 or so) , so I adjusted it to ~45 thou. But no lift kits, no cut springs, and as I mentioned, the whole FP assembly is brand new never used before.

 

[memilanuk]

Monte, I can let you borrow my Deltronic pin gauges. My 223 set has 25 pins in .0001 increments. I personally don't think that is your problem, but they're yours to use.

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

 

[Ggmac]

Since you have the fired case you can compare the cast neck dia to the fired case , just to verify your freebore #s . If the neck dia is the close your freebore should be also . I don't think the amount the brass shrinks after firing would void the test

 

[gunsandgunsmithing]

Savage action from a 12 LRP. Factory 223 bolt head.

There's your problem.

 

[Ggmac]

No. When the first primer pierced, I checked the FP protrusion. It was a fuzz on the long side (63 or so) , so I adjusted it to ~45 thou. But no lift kits, no cut springs, and as I mentioned, the whole FP assembly is brand new never used before.

When you adjusted it did you also adjust the rear fp Spring screw the amount you moved the front ?

 

[memilanuk]

Since you have the fired case you can compare the cast neck dia to the fired case , just to verify your freebore #s . If the neck dia is the close your freebore should be also . I don't think the amount the brass shrinks after firing would void the test

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.

 

[memilanuk]

When you adjusted it did you also adjust the rear fp Spring screw the amount you moved the front ?

No

 

[Ggmac]

No

That would decrease spring pressure . A little , but ! Probably not all of your problem , especially if the pierced primer was after your adjustment