Home Forums Hunting & Shooting Gunsmithing What causes certain barrels to hit pressure before others

I know barrels vary in speed and some seem to have earlier pressure signs. Some obvious variables include bore diameter, type of rifling, throat and lead angle but what are some lesser known reasons some won't achieve the speed before pressure? I just rechambered a 6.5cm to 6.5x284 and I hit pressure signs with 47 gr h4831 and 140 at 2600 with a 22" barrel. That's creedmoor speed. I'm not sure what's wrong. New lapua brass and 1x fired in this rifle. Tried RL 22 with same result hit pressure at like 2625 with 140s. The chamber is tight but bolt closes on new lapua brass smoothly. The chamber has been polished so after reading about the need for brass to grip the walls as it expands I'm going to add some cross hatching thoroughly clean and try again. Any other thoughts on this issue? I know I'm not going to get super speed from a 22" barrel but has to do better than a creedmoor.

How do you know your getting excess pressure. What's happening. I easily get 2900 with H4831 in any 6.5-284 I r done

Really flat primers, heavy bolt and slight ejector mark

I think you may have a headspace issue. GD

I'm glad to see that someone other than me suffers from the Goose Bradshaw affliction, 'cause like Goose, we feel the need.....the need for SPEED!

The Hodgdon site show a max recommended load on 49.5 grains of H4831 at 2,722 fps.

Yeah I saw that and believe it to be on the conservative side as a lot of guys run 50gr or just over for long range loads. I'm expecting a little more out of this 22" 6.5x284. A creedmoor dupes it now

Can you explain? Headspace is tight. Bolt closes on new lapua brass and won't close on brass with masking tape on case head. As long as sized brass chambers can the chamber be too tight? My thoughts are that the tighter chamber will allow less brass stretch and extend life.

I think you don’t have enough barrel length to take advantage of the larger case. What happens when you load the 6.5x284 with lighter bullets using faster powder? I have an old 243 Sako with a 22 & 3/4 inch barrel and I could never get published velocity out of it wether factory or hand loads. 2960 was listed as factory. I had a 243 ai built with a 24 inch Douglass barrel and I blew primers at 3400 ft per sec with 100 grains. I backed off to 3200. Had a Sako 270 years ago and could not get published velocity out of it.

How far from the lands are you seating your bullets???

My brother has a new 6.5PRC that shows pressure signs 4grs short of max from the 2023 Hodgedon book (it's a $2500 rifle).
Every factory load has flat primers. The firing pin isn't hitting the primer dead center. I suspect that the chamber is off a couple of thousands.
Is the firing pin hitting the primer dead center?

Do you know the specs of the reamer used, there's reamers for win. Brass and Lspua.
If a reamer got used designed for Win brass and your using Lapua you could have a neck clearance problem measure fired necks vrs virgin brass or see if a bullet will slide in the neck of your fired cases easily

The brass gripping the chamber walls is a myth. I tested even dipping a cartridge in oil. Same pressure on bolt face.
Charlie

The symptoms described are possible signs of excessive headspace (flat primers, ejector mark). You have answered this. It sounds like HS is fine. A tight chamber won't necessarily increase pressure, unless neck clearance is tight.
Charlie is right on the case gripping thing. There are situations where case/chamber wall adhesion can play a part, but not at high pressure in a front locking action. It doesn't take a lot of pressure to deflect a case head by a couple thou, and the bolt doesn't deflect nearly that much.
It sounds like you just hit on a combination that kind of sux. It was probably a pretty good Creedmoor! By the way I had one batch of H4831 which seemed a lot faster burning than it should have been; you might want to try a different powder. GD

Have you measured the bullets with a micrometer? They could be a couple tenths (maybe more) oversized. This is presuming you are using the same bullets for all loads .

I have 4 rifles chambered in 6.5-284, 3 short actions with 24" barrels and 1 long action with a 22" barrel. The long action rifle with the 22" barrel, a Winchester 70 XTR FWT rechambered from 6.5x55, is slightly faster than the other 3 rifle with the same loads. Why? I honestly don't know why, but it is consistently faster.

I cut the chambers on the 3 short action rifles with the same SAAME specs Clymer finish reamer that I bought new and only cut those 3 chambers. The Winchester was rechambered by Ray Montgomery about 30 years ago with whatever reamer he either owned or rented.

Assuming all your barrel are of the same material, maybe the lead angle on the reamer on the fast barrel is different.

Lead angle can certainly affect pressure. And different angles affect different bullets in different ways.
Charlie

Could be, I've never bothered to make a Cerrosafe chamber and throat cast.

The Winchester barrel is CM and the other barrels are SS.

Three of them are recycled barrels, the Winchester 70 6.5x55 and two Remington 700 260 barrels. The other barrel is a Shilen that I bought new and installed on a Savage 99 CD action.

The Winchester barrel came from the factory throated for 160 grain bullets, I've shot 156 grain Norma and RWS factory loads in another 70 XTR FWT, while the Clymer reamer cut a shorter throat that is more in sync with the short action magazine COAL limits. I have only loaded 90, 100, 120, 125, 129, 130, and 140 grain bullet for this cartridge, so the shorter throat hasn't been an issue.

Do bullets easily fit back into the necks of fired brass?
Maybe inadequate neck clearance

Thanks for the replies. I'll be looking more into this one maybe tomorrow and let you know what I find.