Neck Tension and Pressure Signs

[Bo Ployhar]

We don't clean our barrels until they stop shooting good, but cleaning the chamber properly, and often is a must IMO. As stated above 45 cal pistol jag and 1 or 2 patches on it (what ever it takes for a tight fit), and some rubbing alcohol makes quick work of it. Usually just a few wet patches till clean followed by a dry patch.

I had a batch of new 308 brass that did this a few years back. Some weird polish residue left on the brass. You couldn't wipe it off, but within 10-15 rounds after cleaning the pressure signs where back.

Also make sure you are keeping your lugs greased, doesn't take long to fuck up an action with dry lugs and lots of bolt thrust!

 

[918v]

If your coal is 2.820” then you can’t be .025” off the lands.

 

[OREGUN]

If your coal is 2.820” then you can’t be .025” off the lands.

If it was a factory chambered R700, you wouldnt be anywhere near them

 

[Timo Turl]

Is your factory ammo also using large rifle primers like your handloads ?

 

[918v]

I did a ladder this morning from 41.4-42.7grs in FC brass that weighed 171-172grs, about 3-4grs heavier than Peterson. You have major issues with your stuff. 41.4grs made 2650 FPS in my 24” barrel. No heavy bolt lift at any point, even at max. And FC brass is soft. I did get an ejector mark on top but this gun makes ejector marks earlier than other guns cuz the hole has rough edges.

So it’s not a characteristic of the powder in heavy brass. It’s got to be something in your equipment.

 

[Ratchet 94]

I've been struggling in my 6.5-CM loads with pressure signs (heavy bolt, extractor swipe marks, etc.) in loads that absolutely should "not" be delivering them. I'm talking about velocities that are hundreds of fps slower (2400-ish) than factory ammo (2700-ish) ... that by the way ... shoots with no pressure signs whatsoever. I was at a match this weekend whining about this, and an experienced shooter (that won the match) told me to loosen my neck tension and see if that helped. I'd never thought about that, but it does make sense that if the bullet is too tight in the case mouth, it could build up internal pressure that causes pressure signs but doesn't translate into higher projectile velocity. I size necks with a bushing tie, and then expand prior to seating with a L.E.Wilson Mandrel Die ... but that die is specifically and only equipped with a generic 6.5 mandrel. Assuming it's money well-spent, I've ordered the 21st Century die and mandrel kit to give me the ability to precision size my case mouths (the kit has a range of mandrel sizes over and under the standard) ... and see if I can solve this problem.

Anybody here agree ... that excess neck tension can cause pressure signs with low velocities? Just curious what the heavy-hitters here think about that. And ... am I on the right track with the 21st Century mandrel kit? Thanks! This whole journey is about continuous improvement ... at least that's what I keep telling myself.

I had Lapua brass that the head was not flat , causing heavy lifts every now and then on light loads as I was working up loads. Would scrap brass around primer but not show any other sign of pressure. Filed case heads flat with a smooth file and problem was gone and has not returned. Acted like a hit load but no ejector marks , no flattened primer. Intermittent heavy bolt lift was the symptoms.

 

[ReaperDriver]

I've been struggling in my 6.5-CM loads with pressure signs (heavy bolt, extractor swipe marks, etc.) in loads that absolutely should "not" be delivering them. I'm talking about velocities that are hundreds of fps slower (2400-ish) than factory ammo (2700-ish) ... that by the way ... shoots with no pressure signs whatsoever. I was at a match this weekend whining about this, and an experienced shooter (that won the match) told me to loosen my neck tension and see if that helped. I'd never thought about that, but it does make sense that if the bullet is too tight in the case mouth, it could build up internal pressure that causes pressure signs but doesn't translate into higher projectile velocity. I size necks with a bushing tie, and then expand prior to seating with a L.E.Wilson Mandrel Die ... but that die is specifically and only equipped with a generic 6.5 mandrel. Assuming it's money well-spent, I've ordered the 21st Century die and mandrel kit to give me the ability to precision size my case mouths (the kit has a range of mandrel sizes over and under the standard) ... and see if I can solve this problem.

Anybody here agree ... that excess neck tension can cause pressure signs with low velocities? Just curious what the heavy-hitters here think about that. And ... am I on the right track with the 21st Century mandrel kit? Thanks! This whole journey is about continuous improvement ... at least that's what I keep telling myself.

I know this is an older thread, so apologies if this has already been addressed..... but I ran into the exact same thing recently while doing load development for a .260 gas gun. I was getting pressure signs for no reason on what should have been light loads. Flattened primers, swipe, Ejector hole imprints, etc but at normal velocities using Lapua brass.

After much head scratching, we eliminated it to neck thickness / tight match chamber. The tell-tell sign was that you could not slide a bullet into a fired case mouth. In a normal chamber, the neck obviously opens up to the dimensions of the chamber and you should be able to slide a bullet into the mouth with minor resistance. We also measured the fired case necks and they just barely more than a loaded neck measurement. So we neck turned all the brass to shave a very small amount off and then retried them. Viola, no pressure and the accuracy improved significantly. It was not so much the Lapua brass was too thick, it was the chamber was cut to very min spec and there was no room for the brass to expand and release the bullet during ignition - thereby causing pressure to spike. Hope this helps.

 

[OREGUN]

I do remember some old time Benchrest shooters I used to shoot with talking about going through a phase where they were all convinced that the tightest possible neck would be the best. As it turns out they all found out that’s not in fact the case. You do need some space for the brass to expand and release the bullet. What I didn’t remember about that conversation with them was talking about overpressure or pressure signs but it makes sense. I just assumed they meant their accuracy suffered.

 

[918v]

In some applications you can get away with a tight neck. In others you can’t.