Gentlemen,
I have a 303 sporter built on a 1893 Dutch Mannlicher action by Alex Martin that has seen a lot of use. With even light loads (37 gr 4064 and 180 gr bullets) I get back thrust showing up when I open the bolt. Headspace seems fine, .003, and no unusual case expansion or other signs of pressure. Cartridges and chamber are cleaned of oil prior to firing. Due to the age of the rifle the chamber is quite smooth. Would it make sense to lightly buff the chamber with course abrasive, 80-120 grit, to give the cases something to grip when they expand on firing? Wish there was a good gunsmith in town!
Thanks.

If everthing else is fine, how are you judging excessive backthrust? Several of the older actions flex enough on firing to stretch cases significantly. It's just the nature of the beast and you get one or maybe at most two reloads before case separation manifests.

I have some 1917 303 with Cordite and cupro-nickel bullets that would like to separate on the first firing in a Mk III.

Thanks for the reply. The bolt is stiff to open, this action has forward bolt lugs, unlike the SMLE, so I don't think cases should stretch that much.

Sure sounds like set-back. Could the camming surface be rough?

The camming surfaces of the bolt lugs are extremely smooth with no signs of scratching or wear points. Even when chambering a round only very light pressure is required to close the bolt

It could be that the locking surfaces themselves have set back. With the bolt closed on an empty case run a strong rod down the barrel. While applying pressure with the rod see if you can feel a drag as the bolt opens.
Good Luck!

I've had several '92 and '93 Mannlichers and while they can be picky about ammo, they do usually work well.

When you go to open the bolt after firing, does it turn relatively freely for 20-30 degrees then hang up, or is it hard to even get started?

Will the bolt close OK on a fired case? If so, is it then also difficult to open?

Your load certainly seems mild enough. Are your cases trimmed to length before loading?

The ones I've had have been .256 or .375 2-1/2", never had a .303 (yet...)

hawkins/bpesteve,
Thanks for the thoughts. I tried working the bolt as hawkins suggested with some pressure on a stiff cleaning rod down the bore into a fired case and found that as long as the firing pin remained cocked the bolt opened smoothly with relatively light pressure. When the firing pin was released to the fired position the bolt opens smoothly until the last few degrees of upward lift then hangs up, much as bpesteve suggested. Fired cases chamber easily and smoothly and are properly trimmed. Is this starting to sound like a bur or detent in the camming surfaces that retract the firing pin?
bpesteve, if you still have the 375/2.5 hornadys new 405 Win. brass easily converts to that caliber and is alot cheaper than the aussie stuff. I had to take about .012 -.018 off the rim diameter and .008 off the rim thickness in order to feed thru the 5 round clips but after that they size and trim to length very nicely.
Thanks again guys.

Hmmm... a bit of a poser, this.

The cocking cams are the mating surfaces on the rear of the bolt body and the front of the cocking piece. As you rotate the bolt open, the cocking piece is kept from rotating by the sear nose that runs in the channel in the action tang, and the opposing cam surfaces force the cocking piece rearward along this channel until the bolt body rotates completley in front of the sear nose, cocking the bolt. The bolt body is also turning against the bolt head, though no camming motion occurs there. Cocking cam engagement and action should not be influenced by axial load on the bolt face, although it might be possible that the bolt head may have irregularities where it bears against the front of the bolt body that might hang it up under load.

The other set of cams that come into play manage extraction. These are angled surfaces on the locking lugs and corresponding lug recesses inside the front action ring. These cams only come into play after the bolt is turned up quite a ways, and it is certainly possible that a burr or setback on the cam surface in the lug recess _would_ affect the final rotation of the bolt on opening under axial load. The fly in this ointment is that if this were the case, it shouldn't matter if the bolt were cocked or not.

You say that putting an axial load on the bolt with a rod down the bore on a fired case, the bolt only hangs up if it is both cocking and extracting, not if the bolt is already cocked.

I'll have to take another look at mine to see if I can come up with an idea of how this might happen.

And as to brass for the .375, I got a batch when Bertram first started up many years ago. After soting out the rim diameter and thickness problems they've served pretty well. Sure made me wish I had a small brass lathe, though!

bpesteve,
There is also a degree of stiffness the last few degrees of bolt lift without the axial pressure of the cleaning rod and the firing pin in the fired position. Works smooth as butter when the firing pin is in the cocked position which now leads me to think of a hang up in the mechanism camming the pin back to the cocked position. I'll have to look around and see if I have the instructions for breaking down the bolt and see if I can see any obvious wear. Does this sound reasonable?
Thanks for your time.

Sounds reasonable to me.

To disassemble the bolt, remove it from the rifle then un-cock it by grasping the cocking piece and rotating it counter-clockwise down the cam. This will be under mainspring pressure, but it's easily done and no parts can fly off!

With the bolt un cocked, the bolt head can now be pulled stright off the front of the bolt - theoretically, at least. I've found that most are cranky about coming off, but with the bolt uncocked there's nothing holding the bolt head on. It is not under sping pressure with the bolt uncocked.

It's a bit more tricky to strip it further, but not bad. Hold the bolt body in a way that you can press the now exposed firing pin against a block of wood, while at the same time using the thumb of that hand to draw the safety in toward the bolt body. This will allow the safety to clear the small cut-out on the cocking piece nut which you can then unscrew off the back end of the firing pin rod. The trick is that all this must be done while pressing the firing pin against that wood block under full mainspring pressure. Once the nut is removed you can slowly release the mainspring pressure and all the parts will be loose.

It's more easily done than said - after you've done it once it's a piece of cake.

While the bolt is out you might want to take a peek at the extractor cam surface inside the lower lug recess in the receiver as well.

It's also worth noting that the extraction cam and the actual locking surfaces are completely independent. It would still be a good idea to get a dental mirror or some other tool to take a look at the locking surfaces in the receiver ring too, as hawkins mentioned.

Not having one of these actions to look at I can only guess; The fact that the drag is gone when the firing pin is cocked suggests that somthing is binding during cocking. Take a look at all the surfaces. Don't try to remove metal until you are certain, it" very hard to replace.
Clean things, lightly lube and try again.
Good Luck!

Guys,
Thanks for the suggestions, it'll be a few days before I have the time to check things out but wanted to let you know that I appreciate the time and thought you've given. It's an interesting old rifle that's seen a lot of use but I'd like to get a few more years, and maybe a deer or two, out of it.
Thanks.