I've been dealing with pressure issues in a new barrel at low charge weights and in factory rounds. My chamber specs appear to be good but headspace is at .0065 . To avoid brass flow and case mouth constricting, should I consider reducing the headspace?
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Gunsmithing What do you consider excessive headspace?
- Thread starter SB545
- Start date
Headspace in excess of SAAMI max is excessive.
Figure it out for yourself using the official SAAMI chamber specifications.
Give some more context. Factory ammo or handloads? What do these "pressure issues" look like? How are you measuring headspace?
Most bump the shoulders around .002" after being fired. Sometimes it takes a second firing to really see what you have.
Also, what does headspace have to do with constricting the case mouth?
Most bump the shoulders around .002" after being fired. Sometimes it takes a second firing to really see what you have.
Also, what does headspace have to do with constricting the case mouth?
What is "headspace is at .0065" ???
How did you get that number.
How did you get that number.
Both factory rounds and low book charge weights produce stiff bolt, ejector marks, swipes and case necks that need a chamfer to get a new round in without a press.
I've previously measured headspace with my go gauge and .003 tape on the back that allowed 1/4 bolt throw before stopping. When seating a spent primer in new brass with the rifles bolt I'm getting .0065 . I've tried to keep tolerances on the low end and don't think I'd see a benefit to max headspace of .010
Not uncommon for virgin brass to run a little short, even in a chamber that is at minimum SAAMI spec. The other stuff you're talking about is starting to sound like the neck of the chamber may be tight. A bullet should fall right through the neck of fired brass.
That's why I'm searching for other ideas. I've run new brass, factory brass and brass fired multiple times. I get no significant pressure signs when I charge below minimum. Even when I'm seeing pressure the bullet is slow by almost a couple hundred fps. The higher the pressure the tighter the curl over at the mouth.
Based on chamber casting I have .008-.01 between the neck of loaded rounds and the neck of the chamber.
Shoulder clearance of .006” is fine. You have plenty of neck clearance. Are you jamming the bullets? Heavy bullet, fast burning powder, jam… pressure signs.
That is unlikely to be changed by headspace. I've got a .300 PRC I chambered and it ended up with .006" headspace (tape on the go gauge measurement). The brass shoulder grows by .006" on the first firing but it shoots very well even growing that much.
I'm not sure what you mean by curl. It sounds like you're describing a case that needs trimming but that doesn't sound right either with you saying that's after firing.
Have you looked for anything weird in the chamber/throat with a borescope?
The lands are close but I still have .020 jump. Light for caliber copper projectiles with fast burning powder is what I've been using. I've also tried some typical weight accubonds and some factory soft points.
I've had a look with a low grade scope and nothing jumped out at me.
As for the curl, it's as if the tip of the neck has rolled over to the inside. Imperceptible to my eye but I can't get a bullet down it without a press. If I chamfer the mouth of the case it opens up and I can push a bullet into the case.
Slight reduction of case mouth dia is normal. Not all brands of ammo have cases that are annealed to the same degree. The harder the neck the more curl in you get.
Custom 7 twist .277 sammi spec 270wsm on a Tikka.
.3080 loaded neck diameter on virgin brass. .3060 on brass with multiple firings. Factory ammo is .3070, as it should be.
.3140 od on fired brass. Inside diameter varies if measured before chamfer. Chamber cast measures .3160 at the neck.
Fired ID is near .2800 on brass with no pressure signs and exactly .2770 on factory ammo and loads with swipes.
I thought the same thing at first. The factory brass is the longest I have @2.10 after firing. The reloads are all sub 2.090.
I'll offer this:
A trick that I've used for a long time to see where I'm at on a headspace value is with a small snip of .03" or .04" diameter solder typically used for circuit board stuff.
You cut a piece roughly 3/16ths in length and stick it to the bolt face with a dab of grease. Toss your GO gauge into the chamber and chase it with the bolt. Roll the bolt into battery (use some grease on the lugs as well because it will take a little effort to smoosh the solder and get it to close. Now pull it back apart. The solder will smash into something resembling a miniature bowtie. Measure the "smooshed part" with a mic, and now you have a dead nuts dimension on just how much clearance there is between the head of the gauge and the face of the bolt. In my shop, the term we use is "GO plus, "whatever the thickness is."
Works great. Enough to where we really don't even order/use NOGO gauges anymore, as this gives a value vs. just a dumb pass/fail kind of answer.
Good luck,
C.
A trick that I've used for a long time to see where I'm at on a headspace value is with a small snip of .03" or .04" diameter solder typically used for circuit board stuff.
You cut a piece roughly 3/16ths in length and stick it to the bolt face with a dab of grease. Toss your GO gauge into the chamber and chase it with the bolt. Roll the bolt into battery (use some grease on the lugs as well because it will take a little effort to smoosh the solder and get it to close. Now pull it back apart. The solder will smash into something resembling a miniature bowtie. Measure the "smooshed part" with a mic, and now you have a dead nuts dimension on just how much clearance there is between the head of the gauge and the face of the bolt. In my shop, the term we use is "GO plus, "whatever the thickness is."
Works great. Enough to where we really don't even order/use NOGO gauges anymore, as this gives a value vs. just a dumb pass/fail kind of answer.
Good luck,
C.
Wouldn't excessive headspace make less pressure and more brass stretch ? I've always used scotch tape on the back of the go gauge to measure headspace but remember to peel it after measuring or you will be off on the next chamber job .
You would think that huh? But it does not. I had a ton of issues with mildish loads in undersized brass. Took about 100rds of trying to figure out what the hell was going on before realizing the brass I got was sized too short (for my chamber anyways) and caused a lot of hard bolt lifts and a couple pierced primers. Brass was about 8k too short.
I guess slamming the case back into the bolt would cause hard bolt lift and show pressure signs on the primers .
False pressure signs. Not actually signs of excessive pressure.
I've said this many times here, maybe one day it'll become common knowledge. Excess headspace does not cause higher pressure (quite the opposite), but it does create false pressure signs because the case head is slammed into the bolt face under pressure.
Be careful not to interpret those things as high pressure signs; gotta know what you're seeing but also why it happened.
Another point that seems hard for some guys to understand (or believe) is that the go gauge ONLY matters if you're shooting factory ammo.
When you're loading your own, the Go gauge is meaningless; the only measurement that matters is the difference between your sized brass and your chamber, a reference to some listed book spec has nothing to do with it.
Adjustment of the sizing die controls headspace in handloads. Forget about reference to a gauge.
When you're loading your own, the Go gauge is meaningless; the only measurement that matters is the difference between your sized brass and your chamber, a reference to some listed book spec has nothing to do with it.
Adjustment of the sizing die controls headspace in handloads. Forget about reference to a gauge.
Your right that the brass can be sized to an short or longer chamber to a point but extremely short chamber may not allow factory ammo and a over size chamber cause the same issue with factory brass slamming into the bolt. You have to think about the next guy that ends up with the rifle not knowing how its chambered.
Maybe you should read what I said again, carefully.
Adjustment of the sizing die controls headspace in handloads. Period.
If you don’t understand that concept and are still worried about reference to a go gauge FOR YOUR OWN HANDLOADS then you’ll continue to struggle.
I understand what you are saying no problem and have adjusted dies for chamber size . The OP asked about adjusting the chamber size to save brass and your right he can adjust his sizing die to resize cases so there's no stretch .
I was talking about adjusting the chamber to some size other than saami spec may cause an issue with a future buyer.
Then why are you trying to argue with me about "adjusting" the chamber size? I said nothing of the sort.
But anyway, worrying about the next owner is a silly thing to do. It's your own rifle, do what you want with it.
SB545, Sorry for being late to the party.
Did you solve the problem? Are you going to try the heavy bullets ?(170+) I have a 1-8 twist in a SA Vanguard. I think I have found the heavies jump too far when loaded to max COL 2.860"
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Not yet. Ive been busy with hunts and put this rifle to the back burner. It shoots fine with reduced loads but I have a good 270win that shoots clovers that already does what this barrel can in standard weights. The barrel builder has been unresponsive after telling me he'd make it right. I'm tempted to spin this one off and go with a different caliber from a different builder. To my untrained eye and basic abilities the chamber cast seems to show things are within spec. That leaves the barrel which looks a little tight based on the cast.
Most of the weatherby rifles have a large jump to the lands and it seems to suit their magnum calibers well. I wish you luck running down your issues. Hopefully that doesn't take too much time to get there.
