375 Rum inconsistent pressure signs

[Earnhardt]

The Cutting Edge bullets I've shot have a seal-tite band, and the seal-tite band contacts the entrance to the throat before the ogive contacts the lands.

Cutting Edge recommends seating their bullets into the case neck such that the seal-tite band is located 0.005", or 0.020-0.025", off the chamber throat. Which places the seal-tite band very close to the case mouth.

In post #3, you stated your bullets were seated with the seal-tite band barely touching the lands. Did you mean the lands, or did you mean barely touching your chamber throat?

Because the distance from the bullet ogive to the lands with the Cutting Edge bore rider style bullets isn't used to determine the recommended bullet seating depths. The recommended bullet seating depth is a distance specified from the seal-tite band to the chamber throat. Which means the seal-tite band is normally almost touching the mouth of the casings.

ok, school me here....

My .375 RUM reamer has .300" freebore...

Cutting Edge states, .005" to .020" off of the chamber throat.

So what is the jump on my round going to be if I seat the CE bullet off of the throat .005"...with a .300" freebore?

Are you calling seating off of the throat, what I call seating off of the lands?

Thanks

 

[Pdvdh]

The seal-tite band will be about 0.305" off the lands if the seal-tite band is seated 0.005" off your throat, in a chamber having a freebore, or throat length, of 0.300".

Jump to the lands isn't pertinent when determining seating depth with the seal-tite band, bore rider style bullets.

Seating the seal-tite band off the throat is completely different than seating it, or the bullet ogive, off the lands.

 

[MagnumManiac]

Just to clarify what @phorwath is saying, the throat starts about .050" from the end of the chamber. This has to be calculated into the equation also.
I had the same sort of issues with my 375-416 Rigby Improved 40°, it has .500" freebore, parallel sided at .0005" over bullet diameter at 3.7505". Running my own solid CNC turned copper/brass bullet the shank is long and the ogive is shorter than most. Never could get my hands on Cutting Edge bullets here.

Cheers.

 

[azarcher10x]

This may not have anything to do with your issue, but I begin having sticky bolt lift on my 300 RUM after about three firings. The solution ended up being a small base sizing die. My theory was, as the brass began to become a little work hardened, there was was no spring back in the case itself. A regular FL die didn't seem to do the job. I don't see how this would affect your changes in accuracy, though.

 

[Earnhardt]

.305" bullet jump...

That's almost as far as, The Fonz jumping a shark...

 

[Taylorbok]

Those CE bullets with the sealtite band and bore rider design are completely different than a cup and core bullet. From what I understand best way to start is with the band up against or very close to the case mouth and work out from there.

What kind of velocity from those bullets in a 375 rum

 

[scpaul]

I'm sure that you've thought of it, but have you tried turning the necks of your brass a few thousands just to ensure that for some reason after the initial firings that the necks didn't thicken up as well as possibly growing which with your free bore I wouldn't expect to be the problem.

 

[archeratlrg]

Very first thing I would check is CBTO on these rounds in comparison to older rounds or notes you have taken. If different then this could account for the issue because the load may have dropped out of a node at that measurement.
I had fits with my 338 Edge, as the barrel sped up, so did pressure and it would go on wild excursions for no apparent reason. In the end, I had to re-chamber the barrel, do away with the turn neck dimension and add +P throating. Pressure excursions stopped after this, but I still wasn't happy and sold it.
It could also be a powder issue, which I wouldn't disregard, some lots are very different to each other.

Cheers.

Hey Magnum Maniac, WHT is this "+P throating I'm reading about. Never heard of it util recently but now seeing the tern thrown about quite a bit.

 

[waxonagain10]

What powder you running and approx what % case fill? are the bullets jammed into the lands or being allowed to jump a ways or somewhere in between? And what do the primers look like on the "fine" rounds…are they pancake flat and cratering?

I've found a little jump or freebore dampens out a loads susceptibility to spiky pressure variations. And with big magnums a 100 percent case fill of appropriate burn rate powder is always a good thing.


8 grains average variation in brass weight on a case that big won't single handedly make this kind of difference.

My thought is that you are at the top end of velocities for your bullet/powder combination. Even if an occasional round doesn't stick, others do. I'd drop down to the next accuracy node.i think you're running a bit too hot.

 

[Paladin300]

I would agree that you maybe at the top of pressure. My 338 Rum Imp. has p+ throating. With a 27" barrel my rifle settles in at 91 grains of H1000 and pushes a 300 grain Berger at 2775 with zero issues. I can push them almost 2900 but accuracy goes to crap. Another thought as others have mentioned would be bullet seating. I have not shot the cutting edge but I do shoot that Flatliners from Warner Tool. Those bullet are made to crowd the lands and don't require as much powder to get similar velocities. If I shoot them loaded the same as the Berger, a thou of the lands, I get pressure signs. I would back off the charge weight and play with the seating depth and see what changes. I am thinking it is a bullet seating issue.

 

[7Rum Slayer]

Those CE bullets with the sealtite band and bore rider design are completely different than a cup and core bullet. From what I understand best way to start is with the band up against or very close to the case mouth and work out from there.

This is how I started a few years back shooting the 155 MTH bullet in my 7RUM, I now shoot the 145 Lazer and started with the seal tight band touching and backed off seating depth from there. I wound up very accurate at .010 thou from the band. This only works if there is room to seat with the band in front of the case mouth, my 7 Rem mag was marginal and wanted the band seated in the case so I went to the Hammer Hunters in that rifle. Best of luck, Jason

 

[Korhil78]

When you did the initial load testing when there were no problems, was it with virgin brass or fire formed brass?

Reason I ask is, you may be at top end of pressure like others have been saying. Now that the brass is formed, it might have thrown you out of the node and you may be at the top end of pressure and it's showing up now.

 

[MagnumManiac]

Hey Magnum Maniac, WHT is this "+P throating I'm reading about. Never heard of it util recently but now seeing the tern thrown about quite a bit.

+P throating is a Shaun Carlock innovation, it doesn't extend the throat per se', but elongates the angles and softens the engravement allowing a gentle run into the full diameter of the rifling.

Cheers.

 

[K.E.C. Darkside]

+P throating is a Shaun Carlock innovation, it doesn't extend the throat per se', but elongates the angles and softens the engravement allowing a gentle run into the full diameter of the rifling.

Cheers.

I'm sorry but that sounds like he just help speed up throat erosion. IMO.