Home Forums Hunting & Shooting Ask The Gunwriters Tight Chamber?

I have been hand loading for about 20 yrs now. For rifles, it's typically 5-6 sessions a year. Never a ton of experimentation, pretty much find a decent load and go hunt. In the four rifles I've loaded for (7x57, 7-08. .280, 7mmRM), I've always found accurate loads at book max or very near (sometimes over for the 7x57 and .280). I never have any signs of pressure like flat primers, ejector marks, stiff bolt lift, loose pockets, short case life, etc.

So for the first time, I have a factory 700 LA re-barreled (a Douglas in .338-06). I am amazed that I start to see flat primers, ejector marks and even a stiff bolt lift at charges that are 3-4 grains below, both published book max's and many "go-to loads" discussed here on the fire.

What causes this? Do I have a really tight chamber on this barrel?

If by tight chamber you mean a very short throat or undersize bore, I suspect that you are on the right track. If it was the chamber itself that was undersized, you would likely be having difficulty chambering loaded rounds.

I would recommend a chamber and throat cast to see what you are working with, how that relates to SAAMI specs, and how to make the best of it.

A tight chamber usually means one cut to minimum size, so there's little play in the case expansion, etc, and the necks might have to be turned to fit in the chamber. But that shouldn't really have that much effect on a load. WHAT loads are you using in it. I've seen a lot of stuff on here and other forums where folks seemed to believe they should get 338WM velocities out of a 338/06. Elmer Keith (who designed the round, initially using 333 bullets) used 60grs 4350 behind a 250gr bullet (which is nore doable if using SC powder), and that load gives 2400 fps, more or less. If you go to www.accuratereloading.com, there are some good loads listed there (and you can drool over the loading set up)..

Might want to have whoever put the barrel on have it checked to see if any problems. could be a tighter than normal bore too, which would cause some pressure problems.

I have a custom 7mm STW and a factory 7mm STW in the same length barrel. The loads I have developed for the factory rifle are way too hot for the custom rifle. I can shoot any bullet 100 FPS faster in the factory rifle.....interesting.

I would remove the ejector plunger and ck to see if there is a sharp burr on the ejector hole. A burr could give you a stiff bolt lift and ejector shine on the rim. However that would not explain the "flat" primers if they are truly flat? Compare the primer of your "hot" loads with a primer of one of your "normal" loads to see if they appear quite different. A chrono would also be a big help.

The ejector plunger is held in place, under spring tension, by a small roll pin that crosses a flat on the plunger. You need to depress the plunger when you remove or replace the pin. For this you need 3 hands, or they sell a small tool to depress the plunger and slowly decrease the spring tension after the pin is removed (the plunger will want to fly out of the bolt face and you'll say "Where did that thing go?"

sansarc is on it IMO.

I have seen Douglas (and some other "tight" custom button rifled barrels) that gave book max velocities with powder charges that are a bit under book max.Yet velocity was "there".

This characteristic is attributable more so to a tight,smooth bore,or tight throat,not to a tight chamber.

Last Douglas I worked with in 7RM gave good velocities with charges 2-2.5 grains lighter than a second rifle with a Krieger in the same chambering.

IME so long as function is good,and cases last through several firings without opening primer pockets you should be GTG

You should invest in a chronograph.

I have had the exact thing happen, except it was a different caliber.

For some reason, the chambering reamer was made without provision to cut a throat when the rifle was chambered. The idea was to use a seperate throating reamer, which is fine if you realize that the reamer you have will not cut a throat.

This is very easy to check. Using a just barely resized case, one that the mouth of the neck is sized just enough to hold the bullet.

Start a flat based bullet into the case, nose first, and chamber the dummy cartridge, turning the bolt down.

The flat base of the bullet will touch the origian of the lands and push the bullet into the case.

Then carefully extract the dummy found and measure the amount of bullet base protruding from the case neck. If the bullet is flush with the case neck, it means that your rifle was chambered with a reamer without provision for a throat. No throat, or a short throat will not allow for any bullet jump before the rifling is engaged, and will send pressures over the top. This is what appears to me is what is happening with your rifle.

The fix is simple--have a gunsmith cut a throat with a throating reamer.

I don't know the exact measurement for the throat length for your caliber, but it should be in the range of 1/4" to 5/16."

Much shorter than this, all the way down to the bullet being flush with the case mouth will cause pressures to increase drastically, even with lower that normal loads.

I much prefer freebore over no throating at all.

Providing some more info may provide a more definitive answer,for instance, what bullet, what load and as was already mentioned, are you shooting over a chronograph? and if so what kind of Vels are you getting? besides or in addition to a tight chamber, you might have a short leade/no freebore and tight bore. How far off the lands are you seating your bullets, It may be one single issue or a combination of them or no issue at all. As was pointed out by BobinNH, it's not uncommon to get to max pressure and vels in your rifle before getting to max book loads.
JMHO,
BD

Bingo.

The velocities are likely to be right up there with the pressures you are seeing.

Thanks for all the tips and advice.

I have been loading 200g Nosler Accubonds (and CT Ballistic Tips). I assembled a dummy cartridge at the beginning and found the bullet to hit the lands at 3.46". Backing off .020 puts me at 3.44" -- Noslers listed SAAMI Max COL.

I have recently purchased a chrony. I had not planned to clock loads until I settled on an accuracy load. However, I'll use it next trip to the range to see where i'm at.

My last 200g load showing a "flatter than normal" primer was 59.0 grains of H4350.

I also test fired a round each with a 225g Accubond and H4350, 57, 57.5 and 58 grains respectively. 57 grains showed a flat primer, 57.5 same with an ejector mark -- I pulled the 58 grain load.

My guess at this time is simply a tight bore as others here have suggested. I have loaded a few rounds each of 55 grains under a 225g AB and 58 grains under a 200g AB. I'll shoot these over the chrony and see what it tells me. Hopefully I'll be at or very close to desired velocity with my 24" barrel (2850 for the 200's and 2700 for the 225's).

I finally got around to trying out these new loads today. The 225's need some work as groups were not there (1.5-2"). The good news is that the 58 grain 200 Accubond load grouped well and primers looked okay. I did not use the chrony as the range was packed. Based on this 5-shot group, I hope the load shows decent velocity.

I also have a 338-06 with a barrel instaled by Douglass if you are seating your bulletts as long as the magazine will alow you are most likely seating them into the rifeling. Mark the bullett with a marker chamber the round then extract and look for marks on the bullett made by the rifeling, if you do you must seat the bullett deeper or have the throught recut.