Pressure issues, need some help

[DHD]

Someone else made the point about relative burn rate better than I already. Regardless of what a chart says, some powders don't look at the charts. In my case specifically, N160 and H4350 in a 6×47Lapua. I was seeing 50 fps more for the same charge with N160 compared to H4350. Sierra's load data shows it at least for the 6×47L.

Still not sure why one would add .005" over "go" though. I want my brass to last and will do what I can to make it so.

 

[Steve Donlon]

From what I have seen, switching from small rifle primers to large rifle primers, you need to back off at least I gr. of powder to avoid pressure signs. I get the same velocity with 40.5 grs with a large rifle primer as I do with 41.5 grs using a small rifle primer with Lapua brass. It's a different caliber and different powders, but the same result.

 

[DLT]

That sounds like a lot, more than the brass should be allowed to grow I'd check the size of your chamber at that location where the head swelled.
Something seems strange, almost as if maybe your lot of brass could also be much thicker than normal
Have you weighed it and compared to what that brass should or typically weighs?

 

[DLT]

IMO that's a big red flag on something about your reloading......maybe your scales are lying to you ?
If all other things are equal what else could it be ?

I don’t know. It’s way hot, don’t know why the difference. If it was something with my reloading I would have figured it out by now. I have used this brass before, I have used the primers before and the powder. The scales I have is an older set of rcbs 502s.

 

[M-61]

You set the headspace up .005" beyond the go gauge.

Why?

 

[DaveTooley]

Could anything catastrophic have happened to my bolt or bolt head on my origin ? Is there anything I need to look for ? I am for sure going to take the rifle apart and use the go gauge to get headspace at the bare minimum like I always do except on this job. I did all my measurements after it was installed using a piece of brass and thought it should be fine

Your action is fine.

 

[DLT]

That, I think, is a great indication of the difference in weight of primer compound between large and small rifle primers, and the effect that the mass of primer compound has on velocity, or it's equivalence to grains of powder.

Your mass of powder is relatively small, a large rifle primer adds a substantial percentage of "powder power" to the burn, and accelerates the burn rate, over a small rifle primer.

My suggestion is to come down a full 10 percent from the small rifle primer load, and retest, working back up. This also irons out any issue of the current brass having less case capacity than the old.

The expansion of the case head on virgin brass should be around [correction] 4/10 thou in a small case, that means you are close to max pressure. On RUM brass, I take it to 8, allowing many firings. This 30 [ are you sure it's 30 /1000 and not 3 / 1000 ] tells you there is excessive pressure, not just a little more.

Addendum : 30 thou case head expansion on a 308 size case is impossible, I think.

Excessive pressure creeps in when the cases are too long.

I'd also guess your brass needs trimming. Measure the case length of the fired and full length sized cases. Have to be full length sized. Ask me how I know and how long it took to realise.

Well I have no chambering issues, my sized cases are 1.928 some are longer so they are definitely long fired looks like 1.936 as I only have a couple to look at that. I can chamber a few afterwhile and look with the bore scope and see just how close if any room I have. I know every chamber is different.

 

[DLT]

Your action is fine.

Good words to hear thank you.

 

[SeabeeKen]

The VV data shows a 108 SMK with N-160 max load of 41.2 gr producing velocity of 2920 fps from a 26" barrel 8 twist. You're getting over 3000 fps from a lighter load, possibly from a shorter barrel. This screams over pressure.
1.936 is well over the Lapua trim length of 1.919. Perhaps the case mouth cant release the bullet because it is pinched or jammed into the freebore

 

[DLT]

Someone else made the point about relative burn rate better than I already. Regardless of what a chart says, some powders don't look at the charts. In my case specifically, N160 and H4350 in a 6×47Lapua. I was seeing 50 fps more for the same charge with N160 compared to H4350. Sierra's load data shows it at least for the 6×47L.

Still not sure why one would add .005" over "go" though. I want my brass to last and will do what I can to make it so.

It only ended up like that because I was too hard headed to get a go gauge from my uncle that we use. I used a sized piece of brass.

 

[DLT]

The VV data shows a 108 SMK with N-160 max load of 41.2 gr producing velocity of 2920 fps from a 26" barrel 8 twist. You're getting over 3000 fps from a lighter load, possibly from a shorter barrel. This screams over pressure.

I noticed that in their data. Sierra doesn’t even make a 108 do they ? I assume it is a misprint

 

[SeabeeKen]

I noticed that in their data. Sierra doesn’t even make a 108 do they ? I assume it is a misprint

Your cases are .017 over trim length. Any signs they are jamming past the end of the chamber? Normally .010 over trim length is max length.
Have you borescoped the chamber looking for a carbon ring?

 

[DLT]

Your cases are .017 over trim length. Any signs they are jamming past the end of the chamber?

No sir they chamber with ease. I’ll check again with a sized unloaded case. Found the longest one at 1.936. It chambers fine going off the feel of the bolt

 

[DLT]

Your cases are .017 over trim length. Any signs they are jamming past the end of the chamber? Normally .010 over trim length is max length.
Have you borescoped the chamber looking for a carbon ring?

I have not but I will today

 

[DLT]

I’m going to fix the head space today, I have a box of new lapua small primer brass I have been holding onto as the adg and sig sauer brass had been doing fine.

 

[DHD]

I’m going to fix the head space today, I have a box of new lapua small primer brass I have been holding onto as the adg and sig sauer brass had been doing fine.

Not trying to pick apart your post, but many (most?) times new brass tends to be short at the datum. Check that new brass first before using that as your "go" gauge IMO.

Brownells (Sinclair) makes plugs for measuring your oal case length. You have to sacrifice a piece of brass to get the measurement, but the plugs are reusable. It is a one and done measurement so you can write it down and refer to it for your trim to length. That or just live with shorter than necessary cases. Yeah, OCD stuff there....

 

[DLT]

Not trying to pick apart your post, but many (most?) times new brass tends to be short at the datum. Check that new brass first before using that as your "go" gauge IMO.

Brownells (Sinclair) makes plugs for measuring your oal case length. You have to sacrifice a piece of brass to get the measurement, but the plugs are reusable. It is a one and done measurement so you can write it down and refer to it for your trim to length. That or just live with shorter than necessary cases. Yeah, OCD stuff there....

I have always used a go gauge to set it, I just did not on this latest put together. I will use one today. Probably won’t use brass new or used for this purpose ever again. I know people do I just won’t

 

[dellet]

No sir they chamber with ease. I’ll check again with a sized unloaded case. Found the longest one at 1.936. It chambers fine going off the feel of the bolt

A sized, unloaded case will give a false positive. Note the numbers below, because of the taper at the end of the case neck, an unloaded case could be as long as 1.940 before contact. A loaded round will start to pinch somewhere between 1.9250 and 1.9418” depending on brass thickness. You have a bit of a margin because you set the headspace long. This is probably a good place to start since your trim lengths are not consistent, the problem won’t be either.

 

[big5ifty]

Well I have no chambering issues, my sized cases are 1.928 some are longer so they are definitely long fired looks like 1.936 as I only have a couple to look at that.

Brass that is too long will chamber like there is nothing wrong. There is usually enough clearance to chamber a loaded round, there is not enough clearance where the long case mouth is in the leade to release the bullet.

Is this measured after full length sizing ? The max case length is for a sized case, A fired case is always shorter than a sized case.

Trim to what the book says, AFTER full length sizing.

 

[DLT]

Vv claims a max charge of 41.2 at 2920
With a starting charge of 37.2. Where would you start with new lapua small primer brass and headspace corrected on the rifle ? I usually start middle of the road