Pressure Increase from two Bullets of Same Grain Weight

[BowBaboon]

I started reloading for my 6.5cm with Hornady BTHP Match bullets in 140 grains and using H4350 at 39.6 grains with a COAL of 2.82. I shot about 50 of these and didn't have an issue with pressure marks on my brass. When I reloaded the second batch, I ran out of the BTHP Match bullets and threw some 140 grain ELDM in the remaining 10 with the same charge and primer (CCI 200) and COAL. However, I started to notice slight ejector marks on the bullets with the ELDM. On the 8th shot with the ELDM I noticed significant recoil (in comparison), had a heavy bolt lift, and a definite ejector mark on that piece of brass.

My question is this: would seating the ELDM at the same distance as the BTHP result in a compressed load and more pressure? My thinking is that because the ELDM is "longer" with the polymer tip, the bullet is actually seating deeper and compressing the charge.

Hope this all makes sense. Thanks for the help!

 

[JFK]

Using coal as a comparison is not very useful. Especially when talking tipped and non tipped bullets. You want to get an ogive comparator. Bearing surface of a bullet, and how much of the bullet is sitting below the neck and eating into your case capacity. That’s why new bullet equals dropping down a couple grains and working up.

 

[BowBaboon]

Appreciate the insight!

 

[willfrye027]

you’ve got a few different variables here

Two different bullets with different bearing surfaces (amount of bullet jacket touching rifling basically), they are going to behave differently with pressure

Virgin brass will usually show less signs of pressure. Not uncommon to need to back off a load once brass has been fired.

However it’s not a super hot load..what brass are you using? Are you inadvertently jamming bullets into the lands?

 

[BowBaboon]

The brass is 3rd firing Winchester.

I'm not jamming the bullets into the lands as far as I can tell. I don't remember the specific numbers (written down at home), but I measured using the old school method of placing a sharpied, seated bullet in a fired case and taking measurements off that. I decided to stick to book COAL based on simplicity and good groups.

 

[wind gypsy]

Odd that you'd feel a significant recoil difference in one round. That indicates something may have been up with that one round?

Are you shooting suppressed?

 

[BowBaboon]

I do have a Harvester Evo direct threaded on there

 

[wind gypsy]

I do have a Harvester Evo direct threaded on there

I get inconsistent pressure signs with suppressors in certain weather conditions, most notably in winter with wind in my face. They start after a few rounds. Seems the gas causes condensation or fouling in the chamber that results in cases not gripping chamber walls as well so there's more bolt thrust, ejector marks, and sticky bolt lift. I get it with mild loads and no corresponding velocity increase that to suggest there is higher pressure.

Random test to tell if this is happening - shoot without can for at least 10 rounds first. I thought i was crazy until i did this. There were no issues with 10-20 rounds on a couple rifles prior to putting cans on. Put on cans and before round 10 the sticky bolt lift and ejector marks on brass return.

 

[BowBaboon]

Interesting.

Have you found that keeping the barrel cool reduces that? It was fairly cold out (20-ish F) and I was shooting the first 7 pretty quick.

 

[ResearchinStuff]

That really should be a safe load of powder. If you're not jammed, you might have less jump than you have shoulder setback, and the firing pin hitting the primer might be jamming you. Had that happen this summer.

The other thing that has happened to me giving pressure early is case lube build up in the chamber

 

[wind gypsy]

Interesting.

Have you found that keeping the barrel cool reduces that? It was fairly cold out (20-ish F) and I was shooting the first 7 pretty quick.

I've never tried slow shooting. Even shooting 3-5 before breaking to cool doesn't prevent it. Maybe it would go away if a guy shot one and let it cool to near ambient before shooting again but im not doing that crap.