Pressure Signs - Berger 105gr vs DTAC 115gr

[hacabrera071]

Guys:

This morning I loaded some 6XC rounds as I was testing differences between Berger Hybrid 105gr and DTAC 115gr. All rounds I loaded used the very same brass (brand/model/lot) which had been prepped exactly the same way and at the same time, the only difference is the bullet (Berger vs DTACs) and the seating depth (both Bergers and DTACs seating at .020" off the lands).

The rounds I loaded in a "ladder fashion" as I wanted to see the velocity/grouping differences between the bullets I mentioned above. I started with 37.6gr of H4350, went with .1 increments and stopped at 38.3gr.

What I observed was that Berger bullets were about 30 to 45fps faster (at any charge, that is Berger at 37.6gr vs DTAC at 37.6gr for example) than DTACs. Not a problem, a few fps are not a deal killer for me at this point. But, the part that struck me was that all the cases where I loaded DTACs were giving me a slight "sticky bolt" and the case heads were getting marked (Blue cases had Bergers, Red cases had DTACs), but, none of the cases where Bergers were loaded showed that same issue.

So, my intuition tells me the issue is the seating depth... While both Bergers and DTACs were seating at .020" off the lands and they were using the same powder charge, I'm thinking the DTACs would need to have a different "jump" so as to avoid the pressure signs... Would you agree with this?

Also, if you were to run this test again, what kind of jump would you do with DTACs? A bigger jump (say .025")? or a smaller jump (say .015")?

Thanks in advance!!!

 

[Sheldon N]

You do realize that using a heavier bullet means you need to use less powder, right?

If you use the same amount of powder behind a 115 as you would with a 105, it will be over pressure - as you've experienced. This is a big deal, and sort of reloading 101. This is why they have manuals or online databases full of load data for different bullets weights, shapes, and different powders. Each produces a very different result in terms of pressure/velocity.

An extreme example might be a 308 load where you loaded up 120 grain bullets for varmint shooting and a 308 where you loaded up a heavy 185 grain bullet for long range target shooting. The 120 grain load will have a LOT more powder than the 185 grain load, and if you tried to run the same amount of powder behind the 185 grain bullet you would probably blow up the gun.

A load for a 6XC DTAC is probably 1.5 to 2 grains LESS powder than for a 105 hybrid, to produce equivalent pressure inside the case. Not only is the DTAC heavier, it also protrudes farther into the case taking up case capacity (smaller capacity = higher pressure).

 

[hacabrera071]

You do realize that using a heavier bullet means you need to use less powder, right?

If you use the same amount of powder behind a 115 as you would with a 105, it will be over pressure - as you've experienced. This is a big deal, and sort of reloading 101. This is why they have manuals or online databases full of load data for different bullets weights, shapes, and different powders. Each produces a very different result in terms of pressure/velocity.

An extreme example might be a 308 load where you loaded up 120 grain bullets for varmint shooting and a 308 where you loaded up a heavy 185 grain bullet for long range target shooting. The 120 grain load will have a LOT more powder than the 185 grain load, and if you tried to run the same amount of powder behind the 185 grain bullet you would probably blow up the gun.

A load for a 6XC DTAC is probably 1.5 to 2 grains LESS powder than for a 105 hybrid, to produce equivalent pressure inside the case. Not only is the DTAC heavier, it also protrudes farther into the case taking up case capacity (smaller capacity = higher pressure).

Sheldon

What you are saying makes complete sense to me, thus, I'm thinking I need to dial it down on powder for DTACs even tho I would only be able to run it then at (say) 2800fps (safely, without pressure signs), instead of 3000fps as I'm doing with Berger's.

Now, completely different topic, with Berger's I'm getting about half MOA group at 100yds, this with SD:8 at 3025fps. If I wanted to tight that group up a little, I would need to play with seating depth, would you agree?

Thanks for the help man!

 

[Dstoenner]

I am running 39.2 H4350 with 115 DTAC in 6x47 Lapua. Velocity is 2950 and I am loaded .025 off the lands. My 105 hybrid load is 39.0. That was where each load tuned to. I never found a point where my bolt was sticky or where primers were cratered. I did testing up to 39.5 grains in each bullet.

I know this is a different case but it is smaller than yours. My barrel is a 30 inch 1:7 twist McGowen with a 700 action.

just another data point for you

David

 

[hacabrera071]

I am running 39.2 H4350 with 115 DTAC in 6x47 Lapua. Velocity is 2950 and I am loaded .025 off the lands. My 105 hybrid load is 39.0. That was where each load tuned to. I never found a point where my bolt was sticky or where primers were cratered. I did testing up to 39.5 grains in each bullet.

I know this is a different case but it is smaller than yours. My barrel is a 30 inch 1:7 twist McGowen with a 700 action.

just another data point for you

David

David

Yes, I've definitely seen people pushing DTACS out of 6XC with well over 39.0gr of H4350... That made me feel safer on the upper 37.x neighborhood, but, apparently my rifle is not feeling it for DTACs moving at 3000fps lol, thus I'm thinking I'll dial down the load for DTACs and retest in a neighborhood where pressure signs don't show up...

I'm running a 26" Bartlein barrel with 1:7 twist rate... The thing that might be a factor here is the chambering on the rifle... I know there are 2 different 6XC Chambers (ST & LT), and I know the LT is preferred for 115gr bullets, but I'm not 100% confident that is the chamber I got... I'm thinking I have the ST version which prefers 105/108gr bullets.

Thanks for the info pal!!!

 

[OzzyO20]

David

Yes, I've definitely seen people pushing DTACS out of 6XC with well over 39.0gr of H4350... That made me feel safer on the upper 37.x neighborhood, but, apparently my rifle is not feeling it for DTACs moving at 3000fps lol, thus I'm thinking I'll dial down the load for DTACs and retest in a neighborhood where pressure signs don't show up...

I'm running a 26" Bartlein barrel with 1:7 twist rate... The thing that might be a factor here is the chambering on the rifle... I know there are 2 different 6XC Chambers (ST & LT), and I know the LT is preferred for 115gr bullets, but I'm not 100% confident that is the chamber I got... I'm thinking I have the ST version which prefers 105/108gr bullets.

Thanks for the info pal!!!

There's actually a ton of different 6XC reamers, and that's been an issue as far as I can tell. The ST and LT are what I've seen mentioned with the Robert Whitley 6XC II reamer. Then there's the CIP reamer, Tubb print, and a bunch of others. You need to find out specifically what reamer your smith used when he cut your chamber. I had an issue where I ordered a 6 XC II LT cut, and got a what I can only assume was a Tubb print in acutality. The Alpha 6XC brass showed pressure with virtually anything I tried, as they designed it around the 6 XC II prints, and made them a little big for the other reamers. Sold the rest of the new brass after trashing the ones I'd shot and went to Norma. No more pressure signs (until I hit real pressure) and I also picked up Tubb's 6XC FL sizer. It's a beautifully made die and small bases brass to avoid any issues. I didn't have any alpha to try at that point, but I remember Andrew at Alpha telling me you can small base their brass if you have a different chamber cut, but again I never tried it so I'm not sure what kind of results you'll see.

Tubb sells a brass sampler pack with Norma LRP, Peterson LRP, and Peterson SRP. You could order that and see if your chamber likes one better than the other.

As for your powder charge, you aren't in any crazy territory with the 115's. Do note that the guys you see stating they are running 39.5 are running the boron coated 115's. If you are running the non-coated ones you need to back off at least 1 full grain of powder. I have good results in the 37.7-38.0 range depending on the lot of powder I've used.

My guess here is a chamber/brass issue more than anything.

 

[DaveM]

The base of the Alpha 6xc cases is too soft. You’ll find the base expands twice as much as Norma or Petersen brass when fired. It also the reason for your sticky bolt lift. The 115 has a longer bearing surface and thus generates higher pressures with the same powder charge.

Try the same test with Petersen brass and you’ll be pleasantly surprised.

 

[LawnMM]

Apparently I don't have to respond to anything anymore because Sheldon just says whatever I'm thinking of saying

Seating depth isn't your problem, too much powder is