30-06 AI - Weird Pressure Spikes

[OntarioHunter]

If you meant to say .002” longer than trim length, then that probably isn’t your problem.

Below is the SAAMI spec for a 30-06. The lengths on an AI chamber setup by most smiths will be shorter. Because smiths often setup AI chambers on cases instead of gauges, your chamber dimensions will actually usually be .002”-.004” shorter than the CASE’S neck/shoulder junction. The SAAMI spec on that would be a minimum of 2.1016. If the case your smiths used to set that up was 2.1016, and he barely touched it, and everything else was SAAMI min, then your case mouth would be at 2.495”. However, he likely set it up to crush by .002”+ so your case mouth could easily be at 2.493” or less.

How does a bullet drop into those fired cases necks? They should drop right in. If there is friction on the bullet, the neck is too thick for the chamber, or, if the friction is only at the very edge of the case mouth, then they could be too long and are getting crimped on the bullet when you chamber them.

I am, relatively speaking (re this crowd), a novice and learning a lot from this thread (which is a helluva admission coming from a professed know-it-all). How about simply dropping the reloaded shell into the chamber by hand to see if it binds? Should it fit with light finger pressure?

I'm loading a standard 30-06 with 4350 and finding the casing is pretty full at Hodgon's "medium" recommended powder charge for 165 gr bullet. I checked my powder weight from Lyman electric dispenser against RCBS balance scale and it seems to be spot on. Should I be concerned? Sorry for the hijack. Maybe I should start another thread if there's no fast answer.

Edit: Thanks. Looks like my first question was answered while I was writing it.

 

[JLS]

I'll try to drop a bullet into a fired casing, especially on one if I get similar fliers like what prompted this thread. I'd think, though, that chambering a resized casing would produce a measurable result if it's butting against the end of the chamber.

If you’re chambering an empty casing, then no. It wouldn’t take much resistance to roll in that .001 of neck length.

Have you double checked your charge dispenser?

I’m still in the camp of a brass issue.

 

[Brian in Montana]

So how is a good way to determine the length of the firing chamber?

 

[ImBillT]

Because AI usually shorten during forming, and then they don’t grow as quickly cartridges with more taper, I really didn’t think this was your issue, BUT I think case length is probably it.

Whoever brought that up before I did most likely got it right.

 

[JLS]

So how is a good way to determine the length of the firing chamber?

Have you talked to the gunsmith?

 

[ImBillT]

So how is a good way to determine the length of the firing chamber?

Casting.

I wouldn’t bother. As a general rule for SAAMI spec cartridges you should trim your cases to .010” under max case length and re-trim when they get close. For an AI I would consider .010” under the parent cartridge to me your new max, and would trim .020” shorter(.010” under the new max) That just gives you more room for safety with a shorter chamber. Usually by trimming to .010” under the parent cases SAAMI max, and because AI shrink when forming, people don’t run into trouble. When a smith sets up an AI on a case, he doesn’t necessarily grab one that is .007” shorter than max, AND he may not set it up really tight. But it’s entirely possible to get one that’s .007” shorter, and if he set it up .002” tight, then that would be .009” short, and potentially problematic with cases that didn’t seem too terribly long.

Most SAAMI specs allow for cases to be .020” shorter than max even though the rule of thumb is to trim .010” under.

 

[antelopedundee]

Do overly short cases contribute to carbon ring formation?

 

[ImBillT]

Do overly short cases contribute to carbon ring formation?

I can’t say for sure. It’s not something that would concern me. Use a bronze brush and a decent carbon cleaner. I leave Hoppe’s 9 in my barrels when I put them away. Whether that helps or not I don’t know.

If a carbon ring or a moly ring get really bad, they are supposedly almost impossible to get out, but I’ve never had to deal with either one personally.

 

[Brian in Montana]

Casting.

I wouldn’t bother. As a general rule for SAAMI spec cartridges you should trim your cases to .010” under max case length and re-trim when they get close. For an AI I would consider .010” under the parent cartridge to me your new max, and would trim .020” shorter(.010” under the new max) That just gives you more room for safety with a shorter chamber. Usually by trimming to .010” under the parent cases SAAMI max, and because AI shrink when forming, people don’t run into trouble. When a smith sets up an AI on a case, he doesn’t necessarily grab one that is .007” shorter than max, AND he may not set it up really tight. But it’s entirely possible to get one that’s .007” shorter, and if he set it up .002” tight, then that would be .009” short, and potentially problematic with cases that didn’t seem too terribly long.

Most SAAMI specs allow for cases to be .020” shorter than max even though the rule of thumb is to trim .010” under.

Trimming brass is probably not my favorite way to spend leisure time, but I think I will try this. Trimming them all to 2.474 or thereabouts.

2 things I hadn't considered when trying to test this yesterday - I had assumed the very end of the chamber, where the case mouth would hit if it were long enough, was square, and abrupt stop in a right angle; I looked at some cutaways of rifle chambers online this morning and most appear to have a taper to them. Another thing, the case mouth/high neck of a loaded round is probably a little wider than before a bullet is pressed into it, but guess I really don't know that for sure. I'll have to take some measurements tonight.

 

[p_ham]

Trimming brass is probably not my favorite way to spend leisure time, but I think I will try this. Trimming them all to 2.474 or thereabouts.

2 things I hadn't considered when trying to test this yesterday - I had assumed the very end of the chamber, where the case mouth would hit if it were long enough, was square, and abrupt stop in a right angle; I looked at some cutaways of rifle chambers online this morning and most appear to have a taper to them. Another thing, the case mouth/high neck of a loaded round is probably a little wider than before a bullet is pressed into it, but guess I really don't know that for sure. I'll have to take some measurements tonight.

If you or somebody you know has a borescope you can chamber and empty case and run the scope down the muzzle end. It'll let you see where you're sitting at the case mouth.

 

[ImBillT]

I'll try to drop a bullet into a fired casing, especially on one if I get similar fliers like what prompted this thread. I'd think, though, that chambering a resized casing would produce a measurable result if it's butting against the end of the chamber.

The case was sized down and didn’t have a bullet it in. Your neck internal diameter was less than .308”. It would not necessarily interfere with the chamber the way it would with a .308” bullet in it. A fired case would have come out of that chamber, and thus, would obviously fit, so again, not a good test.

 

[ImBillT]

Trimming brass is probably not my favorite way to spend leisure time, but I think I will try this. Trimming them all to 2.474 or thereabouts.

2 things I hadn't considered when trying to test this yesterday - I had assumed the very end of the chamber, where the case mouth would hit if it were long enough, was square, and abrupt stop in a right angle; I looked at some cutaways of rifle chambers online this morning and most appear to have a taper to them. Another thing, the case mouth/high neck of a loaded round is probably a little wider than before a bullet is pressed into it, but guess I really don't know that for sure. I'll have to take some measurements tonight.

You can also look at the pic of the SAAMI drawings I posted. They will also show you that the case mouth has an angled transition to the freebore.

 

[Brian in Montana]

Thanks for all the advice and comments. I had a little range time this morning, playing around with some 165gn Interlocks and IMR4350. I've trimmed my brass for this rifle back to 2.474" and results seem to be a lot more consistent. I'll admit I was a little skeptical.

This is a bit of a quirky rifle, but I'm figuring it out. Not much freebore, and the chamber does seem to be shorter than expected.

 

[Mtnhunter1]

Glad that it is finally coming around!

Enjoy the range time and that A.I.

 

[cahunter805]

Thanks for all the advice and comments. I had a little range time this morning, playing around with some 165gn Interlocks and IMR4350. I've trimmed my brass for this rifle back to 2.474" and results seem to be a lot more consistent. I'll admit I was a little skeptical.

This is a bit of a quirky rifle, but I'm figuring it out. Not much freebore, and the chamber does seem to be shorter than expected.

Sounds like you are getting it sorted out. Just curious how long were your fired cases before trimming to 2.474?

 

[Brian in Montana]

Sounds like you are getting it sorted out. Just curious how long were your fired cases before trimming to 2.474?

Pretty varied. Most were not long enough to be a problem, but few are it would seem. I haven't paid close attention, but in straining my memory, it did seem to be the longer ones the inconsistencies popped up.

 

[cahunter805]

Pretty varied. Most were not long enough to be a problem, but few are it would seem. I haven't paid close attention, but in straining my memory, it did seem to be the longer ones the inconsistencies popped up.

If 2.474 is working for you as a trim length nothing wrong with that. Just be sure to keep an eye on your cases and trim when needed. I’d say once you hit 2.484 then trim them back to 2.474.

 

[OntarioHunter]

I'm shooting the exact same bullet and powder recipe out of my Springfield 03A3. I also had some inconsistency issues. I finally broke down and rebedded the action Monday night. Got some decent results in a hard wind on Wednesday. I also wasn't holding the gun as tightly as I should have (scope beaned me a couple of times). 53 gr 4350 seems to be working a bit better out of some once fired Sako brass I picked up on that range last time. Incidentally, Federal brass is on average fourteen grams heavier per case and military brass even heavier. Now that I know the bedding is better, I'll try working up to 56 grains powder. 100 yards. Lower target right hole is after windage four clicks right and five clicks down. Last shot is straight above the bull after two clicks left. Then out of ammo. The main object was to see if it would group. It did okay considering heavy gusting wind mostly from about seven o'clock. Shooting off a card table which was not rock solid by any means.

Edit: The holes on the edge of target a from the factory.