Seating depth conundrum...

[daved20319]

So just getting started loading for the 6.5 Creedmoor, and have some lighter bullets left over from my Grendel that still are useable in the CM. Among those is the Speer 90 gr. TNT. But here's the issue. This is a fairly short, flat based bullet, but Speer lists an OAL of 2.7". Doing a little math, that means that the bullet is only seated in the case .122", or less than half the "rule of thumb" one bullet diameter minimum seating depth. I "assumed" (yeah, I know ) that that had to be a typo, and loaded up a ladder at 2.55". My starting load was nearly at the velocity of the max load in the Speer data, and the max was more than 100 FPS faster, and exceeded the max speed recommended for this bullet. No pressure signs, either on the brass or with bolt lift, just blazing fast, and lousy accuracy. No weird velocity excursions, possibly a slight node at about the middle of the charge range, but otherwise pretty linear velocity increase with charge increase.

I was thinking about running some more at the suggested OAL, but I'm not even sure there's enough bullet in the case to hold it. But I guess my question is, could that OAL be right anyway, and if so, isn't that just asking for issues with bullets backing out under recoil? Not really an issue in my case, I run my Savage as a single shot, but it just doesn't feel right. Was wondering if anyone has used really shallow seating depths to good effect. Any thoughts would be much appreciated, thanks.

Dave

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[DizzyJ]

Sounds like it needs to be seated deeper to me. If you're not holding onto it enough with the case neck, it's not going to get a stable launch.

How much of a jump to the lands do you have with that seating depth?

 

[thorborg]

Magazine clearance, headspace, cannelures etc notwithstanding, the general rule to seating depth mechanics is 1 1/2 times the diameter of the bullet as a basic minimum depth inside the case.

 

[osprey]

You need to seat bullet deep enough to keep bullet secure. I would lower charges and redo ladder. If accuracy is still poor with the longer jump to the lands, I would forget that bullet for the rifle.

 

[slimmer13]

I wouldn't bother. I figure out what bullet I want to use, and develop a load that I know exactly where is going to hit. I don't fiddle around with stuff I don't really want to use because I have it. Too complicated to try to keep track of different POI and ballistics IMO.

I'd try to trade them or sell them, but that's me.

 

[Spitpatch]

You need to seat bullet deep enough to keep bullet secure. I would lower charges and redo ladder. If accuracy is still poor with the longer jump to the lands, I would forget that bullet for the rifle.

Yes.

Bullet seating depth "rules of thumb" are quite subjective. ("One and a half" bullet diameter as a "minimum" I would take issue with.) For single-loading bench work (where cartridge durability before it gets in the gun is not an issue), shallow seating depth (to a certain extent) is certainly acceptable. If the cartridge is expected to rattle around in a pocket for a period of time in its life (or be subjected to significant recoil while in a magazine) deeper depth for pre-chambering durability is necessary.

I'm with Osprey speculating toward bullet jump being the detriment to accuracy here. I wouldn't be afraid to walk that bullet out the neck a bit, watching the chrono. The 1/8" (.122) depth seated for a flat base bullet where durability is not a factor I would not be afraid of at all.

Twist might be a factor as well, since just about all the 6.5 anythings are geared for long stuff.

Post Script: Also consider a faster powder than what is customarily used behind a heavier bullet.

 

[daved20319]

Thanks, guys, some good points that I'll think about. Osprey & Slimmer, you're likely right, this isn't a bullet worth bothering with, and I wouldn't other than having a partial box that needed using up, and wanting some practice loading for the Creedmoor before I start using heavier bullets.

For those that asked, no idea of jump, I didn't bother checking, but I'll bet it's too long to seat the bullet out enough to matter. But maybe I'll go ahead and try the last of them at 2.7", just for grins and general edification . And BTW, the powder used was Varget, and is one of the few (only?) cases I've seen where Varget is listed as one of the higher velocity powder choices. Most of the time, I think Varget is just too bulky to get enough in the case to keep up with some of the other popular powders in its class. But again, I'm mostly just using up stuff I have, Varget definitely won't be my powder of choice when I start loading the heavier bullets for long range, but it's one that I have on hand that there's data for with this cartridge.

Again, thanks to all, and I'll post more if I get anything interesting. Later.

Dave

 

[Member 5605]

If your 6.5 backs out under recoil you have another problem.
Neck tension should be sufficient to hold if you aren't trimming too short.

Remember that the deeper you seat it, the higher pressures and more velocity. Same for the inverse. I have some really long seating .308's I can load and I run a full 1.5grains over max and don't hit maximum velocity because the COAL is so long and is fed one at a time.

Sometimes a little deeper can get you to the velocity vs more powder.

The above is not advice or suggestions, just something to think about in the grand scheme of reloading.

 

[orygun]

I'm late to the party, but I'm thinking the long bullet jump is part of the accuracy problem. Take this with a grain of salt as I have no 6.5 loading experience of any kind.
However, I'm not convinced that a bullet that's held in the case with good tension needs one full "caliber" of bullet to case engagement. Many of my 06 loads, both standard and AI don't meet that "requirement" with boattail bullets and still shoot well.
If I was going to use "short" bullets, I'd compromise on both OAL and case engagement. Unless the expander button makes the neck tension somewhat loose, it will work as well as it's going to work. Loading data always has to be adjusted for the components and rifle used.

As an example, the 300 Winchester Magnum neck is too short to give that full caliber engagement and we all know it's worked pretty well for nearly 60 years.

 

[swampbug]

IMHO, why waste your time (and more importantly, primers & powder) on 90gr flat base bullets. The 6.5CM works best with 130 - 140gr bullets. The Berger 130 & 140gr Hybrid target bullets are easy to load for and shoot lights out.