Pressure issue?

[Jefro]

I recently started relaoding 30-06 and have done the normal laddering up on my loads. All cases are trimmed to 2.484" and neck sized only. I'm shooting a Sierra 150 grain HPBT match 2190 bullet seated .015 off the lands which gives me a OAL of 2.790" off the ogive of the bullet and using IMR-4064 powder. Yesterday at the range I shot 20 rounds; five shots each of the following powder charges 48.0, 48.5, 49.0, 49.5. Here is my issue. I am getting an excessive amount of blow back on my case body. It was consistent with all above loads shot. I am assuming that the loads are under pressured and not sealing up tight in the chamber. Am I correct and what do I need to change to fix the problem? I will also let you know that with the boat tail bullets I am only getting about an .125" of contact in the case mouth after seating to the above 2.790" ogive depth. Thanks.

[Clifford L. Hughes]

Dear Jefro:

I don't have my reloading manuals in front of me. Are you loads of 4064 near maximum? I does sound like that the brass is not sealing the chamber. Try a different powder loaded near maximum with a different bullet. I have had good luck with 4350 and a 165 grain Nosler Ballistict Tip. I have found that the Nosler Ballistict Tip as accurate as match grade bullets. If you're shooting gas operated rifle then you must stick to a faster powder than 4350.

Semper Fi.

Gunnery Sergeant

Clifford L. Hughes
USMC Retired

[Jefro]

Clifford, I'm shooting a Remington model 700 and going off the Lyman manual max grains are 53.0 grains.

[Unclenick]

Jefro,

Your numbers don't add up. Did you maybe mean 3.279" COL? If so, I expect one problem may be lack of adequate neck grip on the bullet. You've got around half a caliber into the neck. It may be opening up and letting gas out around the bullet before the pressure gets up enough.

Did you arrive at 0.015" off the lands by testing, or is that just a number you heard somewhere else? I ask, because there is no such thing as a magic good number in this regard. Many bullets need to be tuned in steps to find their best seating depth in a particular chamber. Dave Milosovich wrote in the Precision Shooting Reloading Guide of accidentally turning his seater's micrometer adjustment the wrong way one day and ending up seating some twenty 55 grain Nosler Ballistic Tip bullets 0.050" off the lands for his .220 Swift instead of his intended 0.020" off the lands that he usually used. He decided to fire these short rounds in practice rather than pull and reseat. When he did, instead of getting the usual 3/8" groups the gun had been stubbornly unwilling to improve upon, he had two quarter inch groups and two in the low 1's (bugholes). That chamber and bullet combination like 0.050" off the lands better.

In your shoes, I would start seating deeper. In a box I have of these #2190's I measure the bullet length to average 1.110", the bearing surface seems to be 0.24" long and the boattail 0.150" long, which totals 0.39" seating depth to get the whole bearing surface in the neck.

COL = case length + bullet length - seating depth

2.484 + 1.110 - 0.38 = 3.204"

So 3.204" is the seating depth I would start with, and once I had that shooting as well as it would, I'd then look at nudging it out in small increments.

Your other problem may be powder burn rate. With that small bullet grip the 4064 just may have trouble getting started for lack of start pressure. Try working up some 3031 if the problem continues.

Finally, you'll find it is generally more difficult to get short bearing surface boattails to shoot well than flat base designs. The Berger 150 grain flat base match bullet will give the case more grip and will be possible to get tighter groups with, especially in the 100 to 300 yard range.

Read Dan Newberry's load development system for another approach.

[Jefro]

Unclenick,

"Did you maybe mean 3.279" COL?"
- No, 2.790" is off of the ogive of the bullet.

"Did you arrive at 0.015" off the lands by testing, or is that just a number you heard somewhere else?"
- This is merely a starting point in my testing. Just a number I came up with after researching it.

I think I will load up the next ones to the 3.340" COL and shoot them and see if that helps the problem.

[Unclenick]

Gotcha. Not cartridge overall length (COL, OAL, COAL, etc), but head to ogive length. The problem with that number is that without knowing both the radius of the bullet ogive and the diameter of the gauge hole (they vary by brand), it doesn't provide enough information to ascertain seating depth.

What you can do instead of making the ogive radius determination and the gauge hole size and the calculations that follow, is simply zero the caliper with no bullet or case, then put a bullet in the gauge by itself and say what it then reads. That will reveal how far up from the bullet base the gauge is touching the ogive and then the seating depth details can be worked out. At this point, I still don't know what you were actually seating to.

3.340" is the SAAMI maximum COL to use for a cartridge that is expected to fit in any SAAMI compliant magazine made for it. Shorter is OK, but longer may not fit in or, if it does, may wedge against the front and fail to feed. It has no bearing on what's best for any particular bullet, and the lighter ones often need still shorter seating. It's not uncommon to find more than one seating depth sweet spot for a bullet, though with the short bearing surface ones this is more problematic.