Sticky Bolt and pressure signs, but no excess velocity?

[AKShooter]

Hello

First let me say, this rifle is now back at the gunsmith now, to have them go back through it. (I had another thread here on the issue) I feel something is up, just not sure what. It's a 7saum. custom action, custom barrel, While I'm waiting for verdict from the smith, figured I'd poll the fellas here.

I'm having pressure signs at low powder charges, I'm basically finding MAX at about a grain above published starting loads. Stepping up will produce sticky bolt and ejector marks on brass.

But here is what is odd, My speeds are about what I should be getting at these powder charges, if not lower. Not getting high velocity along with the high pressure...Getting around 2750fps with a load the books estimate at 2800, and thats with a 24 inch barrel, mine sports a 26.

I'm hoping for word back from smith next week, but anyone run into this? I'm thinking maybe something with chamber cut, excess headspace, or maybe a tight barrel? Guns out of my hands, but thought I'd ask here.

 

[bigedp51]

The short and fat ultra magnums are known to have bolt thrust problems, meaning your rifle has a large diameter case BUT not enough cartridge surface area to grip the chamber walls tightly.

With your custom barreled action you might have a tight neck and peak pressure problems. Is your gunsmith also the builder, if so he should have the correct tools to find the actual problem.

 

[BoydAllen]

If you are not getting all of your sizing lube off your cases, and it gets carried into the chamber, your bolt thrust can increase by quite a bit. Try making sure that your chamber is clean and perfectly dry, as well as your brass. Also, a friend would have pressure signs after cleaning because he would reuse a chamber swab to wipe our and "dry" the solvent from his chambers. When he changed procedure so that his chamber was completely lube free, the problems went away. Do you have a hard bolt lift from the start, or does the bolt lift OK but you have trouble at the top of the handle lift. Also, when you size and reprime your cases, how do the pockets feel? IF you are not feeling slightly looser pockets, you may have an issue with bolt thrust that is not caused by over pressure.

 

[Lapua40X]

Might want to check the neck size relative to the size of the chamber. When a neck fits too tight in the chamber there is no place for the neck to expand so the neck tension on the bullet remains greater than it should and the bullet resists separating from the cartridge. That can run pressures up without reflecting velocity differences.

 

[AKShooter]

All great advise guys. I'll see what the smith has to say. I will take the advise to heart. Pay attention to lube and chamber when I get it back. Shoot a couple other mags. A 300 and 375. But not a short mag. Sticky bolt is one thing. But ejector marks on brass raises my eyebrows !

 

[BoydAllen]

I believe that short fat cases have a greater tendency to have this sort of issue.

 

[bigedp51]

Sticky bolt is one thing. But ejector marks on brass raises my eyebrows !

AKShooter

The newer M16/AR15 M855 military amunition is loaded hotter than the older M193 and .223 ammunition. Also military cases are made harder to withstand higher pressures and larger chambers.

When people load civilian .223 cases for their AR15s to military pressures you will see ejector marks and swipes on the rear of these softer cases. In these forums here I have read where some reloaders will increase the load until they get ejector marks on the case and then back off a grain or two and call this the max load for their rifle.

Below is an example of soft Winchester .308 cases fired in a 7.62 M1A loaded a little warm. Also because this M1A is over gassed and the lower right case had a loose primer pocket the primer actually backed this far out of the primer pocket under pressure as the bolt was moving to the rear.



Case-Head Swipe
(I know your rifle isn't a AR15 but read this and what is said about powder burn rates and ejector marks)
http://www.shootingillustrated.com/index.php/8720/case-head-swipe/

Questions

1. What make cases were you shooting?
2. What brand and weight bullets?
3 What powder did you use and what was the charge weight?

Advice: I have been to Alaska several times and seen the Moose and Bears.............................sell the 7saum and get a bigger gun.

 

[amlevin]

If you are not getting all of your sizing lube off your cases, and it gets carried into the chamber, your bolt thrust can increase by quite a bit. Try making sure that your chamber is clean and perfectly dry, as well as your brass. Also, a friend would have pressure signs after cleaning because he would reuse a chamber swab to wipe our and "dry" the solvent from his chambers. When he changed procedure so that his chamber was completely lube free, the problems went away.

A cheap can of BrakeKleen (Generic "house brand" is even better) can be your friend. After cleaning, take a mop and make sure it's lube free by rinsing it in BrakeKleen. Then swab out the chamber.

 

[BoydAllen]

+1

 

[AKShooter]

Well, hopefully I'll know sometime next week, but if I get a clean bill of health from the smith (they also did the work) then I will be cleaning a chamber better! For the last go I did use a brand new chamber mop. But no cleaner

Using nosler, remington and Norma brass. Some has been neck turned. Some not. Bullets have been 150 and 160 noslers. H4831sc and H4350 have been the powders with nosler manual driving charges.

Kind of at the point I at least wanted to make sure everything is correct, rule out mechanical error. I had an anomaly with one batch that let me get close to max without pressure signs. So maybe it's user error. In any case had enough issues I wanted it checked out.

Copper fouling bad after 50 rounds, in a hand lapped barrel, so was thinking bore diameter. Also my fired rounds shoulders are not super sharp. Somewhat rounded. Appreciate all the responses. Adding solvent will be added to cleaning process when I get it back

 

[AKShooter]

Hello, just wanted to give some closure here. Got word from the smith. While not completey positive about the cause of the issue, I do have some aditional bits of info.

Most likely the overpresure had 2 or more items affecting it. First off, headspace was a little long, do not have the actual numbers, but the barrel was turned back to decrease the headspace.

Second thing, I feel, is the chamber. I was not using a solvent cleaner, and chamber was mirror bright. Smith put a bit more "grab" to the chamber with a scotch bright pad. I will add additonal cleaning to my system for the chamber

Anyway, he was able to load a 180 well over what I was using, with velocity right where it should have been, , and just off the lands too... So, with that, I should be able to start back over with my loading, working up of course, can't wait to get it back in my hands!
Can't say enough about my smith for follow through, will use them again in the future

 

[BoydAllen]

Thanks for getting back to us.

 

[rvn1968]

Forget about MV. If you want to see the absolute highest pressure,block your bullet.Nothing beats high pressure signs like a blocked bore.If you start with that in mind,then you think about anything you might be doing to cause overpressure.Work backwards from the problem.IMHO Tom