View Full Version : Throat angle

11-22-2016, 02:58 AM
If you change the throat angle, say from the SAAMI spec of 47' 33" to 1* 30' on a given cartridge, will that increase chamber pressure with everything else being equal?

11-22-2016, 06:13 AM
If you change the throat angle, say from the SAAMI spec of 47' 33" to 1* 30' on a given cartridge, will that increase chamber pressure with everything else being equal?\\

IMO, it would depend on the bullet ogive, the amount of jam, and the powder used. Some powders are still building up pressure a few inches down the barrel.

Probably that small of an angle change wouldn't be measurable.


11-23-2016, 03:05 AM
The reason for this question is that I am trying to diagnose a high pressure problem on a factory barrel that I set back with a tight match grade reamer. The caliber is 270 Win. I set it back the length of the neck plus a little to be sure everything cleaned up. Originally the back of the chamber was on the very large side making it difficult at best to resize the brass. I never shot this rifle much due to this, but what I did shoot were medium to heavy "book" loads that were typical to pressure signs. This barrel is extremely rough and copper fouls badly. So bad, that after 15 rounds and running a few patches of bore cleaner through to get the black out, the bore is solid copper, lands and grooves, not even being able to see any of the factory bluing in the bore. Now I'm getting high pressure signs with even light loads. There is .004 neck clearance, the trim length is good, a bullet will slip into the neck of a fired case without a hitch, the head space is at minimum and the brass is sized perfectly for a light crush feel with a stripped bolt. So I'm here scratching my head trying to decide if the tighter chamber, the throat angle, the shorter free bore or the heavy copper is the culprit. Or maybe it's just a combination of all of these things together. In any event, I had a spare barrel that I installed and things are pretty much back to normal. The old barrel is on the tomato stake pile for the spring. Any useful comments or ideas on why?

P.S. What I may do just out of curiosity is pour a cast of the bore near the muzzle just to check the groove diameter to see if it may be undersize. The groove diameter doesn't appear as rough as the lands but still coppers badly. :confused:

11-23-2016, 06:04 AM
What I'd try, clean the barrel to the bare metal then slug it. If it slugs and measures to caliber diameter, lands and grooves, otherwise I'd try lapping it at least with a tight patch and 200-600 lapping compound. Aluminum Oxide not Silicon Carbide.

Otherwise, since it is a factory barrel don't set your expectations too high.

Spring is just a few months away, I do love fresh tomatoes.


11-23-2016, 07:32 AM
Jerry, in hindsight, I should have probably sent it back to the barrel nut people that made it. But since I cut on it, that is a waste of time. When I say this, I'm not kidding not even one little bit....that barrel is the roughest barrel I have EVER seen on ANY rifle. It is soooooo rough that you can literally hear the patch go zzzzzzzz back and forth in the bore. Railroad tracks are smoother. But hey, lesson learned and I'll add it to my experience collection. Sometimes you just have to learn the hard way. :p

11-23-2016, 10:51 AM
I'd send it back anyway and see what happens. I say that as I've never sent a barrel back to the barrel maker...but should have a couple of times. Not shooting well is one thing but having the problems you're having is another.

11-30-2016, 02:51 PM
I just thought I show a pic of that barrel after 15 rounds. It still amazes me.

11-30-2016, 07:03 PM
About a 15 minute fixable solution IMO.

12-01-2016, 01:25 AM
Ok, I'll bite. Your 15 minute solution is?

12-01-2016, 07:50 AM
Ok, I'll bite. Your 15 minute solution is?

Remove the current fouling. I would use a couple of new 30 cal brushes and Sweets 7.62 keeping the brush wet both ways - about 40 strokes each way.. You may even need to use some JB's (ugh!) on a patched brush.

Buy a box of loaded Final Finish in 270 or buy and load the first 20 (most aggressive grit) in the Final Finish kit. Read the instructions...

Lubricate the clean barrel with a light oil and each FF bullet fired down the barrel with the same.

Follow with about a dozen bare bullets with the same lightweight charge to burnish the barrel.


12-01-2016, 09:22 AM
Mr. Tubb,
I'm not saying that doing what you suggest won't work but, the barrel has already been replaced. Secondly IMO that barrel isn't worth putting any more money, time, powder, primers or fuel for the truck into. I've already put more time & money into that ragged piece of junk then what it was worth brand new. Actually the copper in the bore is probably worth more. I think purely for my satisfaction, I will take it with me on the next camping trip, drive it into the ground muzzle first, with a steel hammer, tie the tent pole to it and when I pack up to come home, leave it right where it is. Or should I drive it the rest of the way below the frost line so nobody trips over it? Hmmmm. Thanks for the suggestion though. I appreciated it.

Jerry Reisdorff
12-06-2016, 09:59 AM
But it would be interesting to see if his solution did fix the problem !


12-06-2016, 12:07 PM
Jerry there is a possibility that it just may. But my solution definitely will without a doubt. ;)

12-07-2016, 09:27 PM
The late Skip Otto useta' firelap barrels with tranny fluid......not to clean anything but just to "lay down" the bore.

He'd mount the rifle muzzle down in a vise and swab the bore with ATF, then tight-patch it with a wetted patch to get all of the liquid out leaving just a wetted bore surface. Then tip the muzzle UP, swab the chamber/neck/throat dry with lighter fluid and fire nekkid bullets.

NO abrasive grit

NO liquid in bore

He felt this ironed the bore down but he warned that ANY LIQUID in the bore would dimple, dent or ring the bore.

All's I know about it, having never tried The Skipper's method, but having distorted some barrel steel is that IME it doesn't take much liquid to ring and ruin a bore.

12-08-2016, 06:58 AM
"Fire lapping", what David describes above, is a commonly used barrel conditioner for hunting gun applications. Don't know about it being used to improve sub quarter-minute barrels. The commonly used path to fix a poor quality benchrest barrel is to send it back to the manufacturer. Bad barrels, even after chambering, that do not preform to reasonable accuracy are sent back.

Example, a few years ago I had chambered a benchrest barrel made by one of our top manufacturers. After shooting in 3 aggs at an NBRSA match it was still coppering in an area about 6" long and in about 4" from the muzzle. Sent it back, they sent me a replacement, no questions ask. This barrel had an oversized spot, apparently where a void had been lapped out. Slugging a new barrel blank is the best pre-shooting test I know.

What Skippy was doing, ATF contains some EP (Extreme Pressure) ingredients that can pre-condition a new, freshly lapped, surface so it doesn't try to grab copper off your hand made bullets. NAPA LokEase, which contains submicro graphite particles is a more common used preconditioner and reconditioner (after cleaning) by many HOF shooters.