I have hesitated to post this before out of concern that I not show my ignorance, but that hasn't worked as reading my posts would indicate. I am the proud owner of a P14 that has been rebarrled to 444 Marlin. I really enjoy shooting it, but the cartridges feed like a dog shitting peach seeds. Even single shot loading is bad. So what cartridge would be easier feeding, and would require just a chamber ream? Or, would a rebarrel to say 416 Something just be the best route? Feel free to make fun, I am looking for some ideas however.

Tell us more about the failue to feed. What is happening . . . or NOT happening!
Was it modified in any way other than rebarrel?
Some of the real and "real" experperts can give you answers with out details, for the rest of us we gotta have some info to go on.
LouisB
Just an opinion of course!

"cartridges feed like a dog shitting peach seeds"

That is pretty good! : )

Hey, it is not the action. It is whoever put the thing together. You need to modify the rails extensively. I have a 45-70 p-14 I made that, well.... feeds slicker than catshit on a linolioum floor, no,no, I got it, whale shit, on a linolium floor!

You need to know how to visualize what is going on and how to change things to make it happen right. A little bevel under the extractor will be a good start, if it binds the rim while the cartridge moves up. The rails need to nestle the cartridges along their length. In other words the body portion of the rail cut need to go foreward. The rim cutout may need to be a little deeper. I did mine with patience and a Dremmil tool. I also modified the follower. I can't begin to tell you on paper how to make it feed. IT is even harder to make it feed form both rails equally well. Maybe find someone who does 45-70 Siamese mausers, same idea.

Thanks for the thoughts! As to the feeding problems, the cartridge rising to the bolt face is not smooth, any hesitation here seems to amplifly feed problems. Also, the nose of the bullet does not like to ramp up. Even if those factors don't come into play, the cartridge likes to hang on the bullet nose on entry to the chamber. That was easily fixed by switching to roundnose bullets, but then the leading edge of the brass mouth will do the same, on the point of entry to the chamber. If it catches just right, ruins the case.

"As to the feeding problems, the cartridge rising to the bolt face is not smooth"

Check the following with a bolt and extractor but no striker. Check the cartridge rim fit under the extractor. IT should snap in the bolt face recess with minimal effort, but not be completely loose. Will the cartridge rim fit in the bolt face recess, with out the extractor? IT should slide up from below. IF it does not open and bevel the bolt face face so that it can. I like little riffers and auger sharpening files here. You need a safe side file. Can the cartridge rise up the boltface with a little angel?? Like when it was feeding?? IF no put a little bevel in the back of the bottom of the extractor. Make changes a little at a time. Think before you remove any metal. Polish everything after your done and rinse all the grit off.

Since you're hoping to just rechamber, and already have a gun with a .429 bore, I think you're in wildcat country.

You might want to look at a standard magnum cartridge with the case shortened to 2.225 and opened to .429, or one of the new "short" mags opened to .429.

I don't think the .350 RemMag opened to .429 will work since its case is 2.170, but it may be worth asking your gunsmith about.