I’ve been working up a load for my new CZ 550 chambered in 505 Gibbs. I’m using Hornbear brass, 600 grain Woodleigh protected points, & Hodgdon 4831 powder. I’ve worked up to 142 grains of 4831 for a muzzle velocity of 2300 fps. I’ve started to notice some circular markings on the case head. I know that when you start to see ejector markings in the case head pressures are way too high, but the ejector cut impressions are absent.

I’ve noticed two things so far in my search for an explanation:

1. The case heads of the unfired Hornbear brass that I have are nicked up pretty badly to begin with.
2. The bolt face of the CZ is rough and sloppy. It looks like they took an existing bolt (for a smaller cartridge) and opened it up to the Gibbs head size. I don’t know what tool you would use to open up a bolt face (cutter of some type?) but there are two distinctly different textures where the original cut was made and where the face was opened up. Upon further inspection I believe one would discover a depth differential between the two cuttings. I think this is what’s causing my weird circular markings.

It’s been made clear to me by the gurus here that 2400-2500 fps is achievable within safe parameters in the Gibbs, so I’m not questioning them. My concern is that am I running at too high a pressure by using H4831 or is it a rifle/component problem? Is the Hornbear brass soft, do I have a head space issue, or am I right about the rough bolt face?

I do have some Jamisson brass that I have not used yet, not sure if it is any harder or not (it is heavier though, so I’ll drop the charge and work back up). I’ve got a Vernier micrometer and will try to measure some of the case heads this time around (not sure if this is a waste of time or not).

I’d like to reach 2400 fps, so if any one can offer some advice it would be greatly appreciated. OBTW, feel free to criticize CZ if you’d like it won’t hurt my feelings . I know I could have got a Hein for only a grand more, but I knew what I was buying when I got the CZ. I just wanted something relatively cheap and readily available to play with.

Thanks for any replies….

Matt V.

It seems the bolt face on my CZ 505 Gibbs needs polishing too.



The grapevine has it that Ryan Breeding's max load for a Granite Mountain actioned .505 Gibbs (.750" bolt diameter) with 600 grain Woodleigh Weldcore is 144.0 grains of H4831, and gossip is that this produced 2500 fps, in what length barrel not stated.

That kind of velocity would turn the Woodleigh Weldcore into a pretty big pancake of bonded lead and copper jacket upon impact with a buffalo.

I do not know if the pressure for that load is over the maker's indicated 55,000 psi limit of the CZ .505 Gibbs (dictated by bolt thrust with the big case).

My feeling is that if you want 600 grains at 2500 fps, it would be safer and easier to do it with a 500 A2 or a 500 Mbogo.

Here are the other Ryan Breeding loads, much more sensible in a .505 Gibbs:
Horneber or Jamison brass
F215 primer
525-grain Woodleigh Weldcore RNSP
H4831 powder
122.0 grains >>> 2100 fps
128.0 grains >>> 2200 fps
135.0 grains >>> 2400 fps

Your velocity seems low for such a large charge. I cannot verify the above loads, just internet gossip.

I have to wonder if sumbuddy has gotten IMR 4831 data mixed up with H4831.

IMR4831 would be a lot faster and higher pressured.

Open for discussion here of anyone who has tried those powders in the .505 Gibbs. I have not gotten around to it. Got distracted by the 500 Mbogo and the various .395's.

RIP,

Thanks for the pic and the data. Makes me feel better knowing I’m not the only one with a rough bolt face. Though your bolt face is even smoother than mine! I’ll post a pick if I can figure out how. Did your bolt face leave any markings?

I have no desire to go to 2500 fps, I’m pretty satisfied with 2300 fps. Yet there is that propensity in me (like most hand loaders) to want to push the envelope a little farther. I’ll probably not try to wring much more velocity out of the Gibbs. Like ya said, the bullet probably would not hold up anyway. The recommended manufacture impact velocity is around 2200 fps. With a muzzle velocity of 2300 fps it’ll drop to 2200 fps pretty fast.

144 of H4831 to yield 2500! I’d like to know what length barrel that was? I’m averaging about 50fps every two grains added. I know it’s a diminishing return but I’d have to go to around 152 grains. I would have a slightly compressed load at that charge. Would H4831 SC make any difference, or should I go to a faster poweder?

Anyway here is some data I got from Geoff McDonald @ Woodleigh:

600gr PP:

110gr of H4350 = 2012 fps
115gr of H4350 = 2085 fps

600gr FMJ:

110gr of H4350 = 2074 fps
115gr of H4350 = 2150 fps
120gr of H4831 SC = 2280 fps

Mr. McDonald said that their test rifle was a Ruger No.1 and that I’d probably be able to go higher in a bolt gun. Imagine touching one off in that combo !

Thanks again..

Matt V.

Matt,
My CZ does leave a little smear marking on the brass headstamp area. This was with a low starting load of 132.0 grains of H4831 and F-215 primer and Jamison brass: I used the A-Square 525-grain Dead Toughs that I pulled from CZ factory loads. Velocity was only about 2200 fps. Pressure for that load is supposed to be around 35,000 PSI according to the A-Square manual.

I will eventually just polish my bolt face with a polishing spud and polishing compound, and touch up that rough spot on the extractor.

The travails of the CZ ...

If you got to 2300 fps with 600 grainers, I think that is fast enough for sure with the Woodleigh 600 grainers. No hype or chrony factors there, I'll bet.

Maybe all you need is a bolt face polishing.

Nickudu did the sensible thing also by stopping at about 2300 fps with his 570-grain and 600 grain loads for the Barnes bullets, using RL-15 around 120.0 grains and 4 to 5 grains of Dacron filler, as shown in the reloading pages of this site. Maybe less recoil than with the H4831 loads. Another way to skin the cat.

I was unable to chamber the initial CZ/A-Square factory loads in my CZ. Could not close the bolt on a single one of them.

The Jamison brass run through the RCBS sizing dies works.

So I ended up returning all but ten rounds of the CZ/A-Square ammo, and CZ came out with a new batch that I will never be trying, at those prices!

FWIW, I pulled the bullets on 10 rounds of the CZ/A-square ammo, 5 softs and 5 solids, and measured the powder charges, all were 525-grain bullet loads, advertized at 2300 fps.

Soft: average = 135.4 grains
Solid: average = 133.0 grains

The powder looked like RL-22.

RIP,

I think you’ve talked some sense into me. 600@2300 produces some nice numbers, 7050fpe & nearly 200lb-f/s. The bullet would be at recommended impact velocity within 40 yards and shouldn’t be too strained within that first 40.

Recoil is manageable now but at higher levels? My CZ is a bit on the light side at around 10 lbs (going to ad more mercury). And the fact that the stock CZ trigger is in the middle of the guard bow doesn’t help when you’ve got large hands (ouch).

I couldn’t chamber new A-Square brass either. After a run through the sizing die it was fine.

Going to have the action slicked up anyway, just will have the bolt face polished now as well. It’s just like ya said, “The travails of the CZâ€.

Matt V.

Matt/RIP - The boltface on mine looks much the same as yours. Too many variables involved to qualify your loading but as I also use Horneber brass, it would be interesting to compare mic readings at the pressure ring, from 1x fired casings to new.

Nickudu,

I’ve got a Starrett (standard/non-blade type) Vernier micrometer. Where exactly do you take the pressure ring measurements, and on the first firing what level should this be done at (142 grains or should I back off for the first firing)? Is .0005†the ceiling I’m looking for?

Thanks for any advice…

Matt V.

Matt - With that type of mic I suggest you simply match up the rear edge with that of the forward edge of the extraction groove, as your surface is certainly wide enough for our purposes. Do so on both new and 1x fired (unresized) brass from that 142 grain loading. Repeat once or twice in different spots. Can you give the differential? Send PM.

my 505 experience is all with 650gr GC cast bullets, imagine the fun turning gascheck shanks on a hundred plainbase bullets...?
I suggest trying 120gr of VihtaVuoriOy N160 and same of N165, and working up from there. The N160 load will run out about 2100, the N165 about 50fps less. Low, yes, but a good safe place to start. Besides, I could shoot the rifle from the bench 8-12 rounds a session. VVo's technician said the factory load was 39,600Psi and the 120gr N160 was an even 40,000Psi; that gives you a reference point. I got up to 2278fps (6-shot string) with the N160, but it was too much fun sitting!

hope this helps,

Rich
NRA Life Member
DRSS

Nickudu..

Thanks for the guidance, I'll try to take some measurements in the next few days..I'll PM you the results..

Rich...

I may try switching powders eventually...
650@2278 wouldn't be much fun standing couldn't imagine it from the bench!

Thanks again…

Matt V.