Home Forums Hunting & Shooting Gunsmithing Chamber Reaming for Ackley-Improved calibers?

I have a couple of rifles that I'd like to have improved -- without replacing the barrels. It's not that I need the improvement, but I'm a dyed-in-the-wool tinkerer, and I just think it would be cool.

What are the chances that I could have an AI reamer run in the chamber of a Weatherby Mark V .375 H&H and a Weatherby Mark V .280 Rem without ruining them? Can anybody recommend an excellent riflesmith for this type of work?

To do an AI correctly will usually require more than running the reamer into the existing chamber.

Okay. Let me rephrase the question: Can I have my .375 and .280 converted to AI without rebarreling?

If so, what is involved?

To cut a proper AI chamber for a rimless cartridge the barrel must be set back since the neck/shoulder junction of an AI chamber is .004" shorter than the parent cartridge.
This is to provide a good crush fit when loading the parent cartridge into the improved chamber.

Rimmed cartridges usually do not required the barrel to be set back.

The .375 H&H, being a belted cartridge, would probably not need the barrel set back since headspace is controlled by the belt.
The .280 will need the barrel set back.

You can of course still use the original barrel for both of these. No new barrel would be required.

I understand that the barrel may need to be set back, but what is the likelihood that the chamber can be reamed concentric with the original chamber? Isn't there some risk with this, or is it straight forward?

Very straightforward.

If the rifles shoot well now they will, in all likelyhood, shoot well after reaming to AI.

Great. Does anyone know of a riflesmith that routinely does this kind of work?

I would probably recut the H&H chamber to .375 Weatherby...jim

I'm interested in the .280 AI as well. My Ruger Hawkeye .280 shoots .5" groups with the factory barrel and my handloads at 100 yds. Should I even mess with it or would reaming the chamber to AI be a good idea?

Factory barrels that shoot .5" groups, I tend not to screw with.

That was my thought as well! The AI might give me more velocity but I'm happy with what I'm getting from my handloads. Thanks for the response!

It'll work. I've had factory barrels rechambered a few times. But there are things to watch for when re-cutting a factory chamber. Factory chambers can be large and a custom reamer will easily be tighter. There needs to be enough setback where the reamer is cutting new chamber back to the web and takes out all factory body taper. Otherwise you get a fired case with two different body tapers. I had that happen.....wasn't much, but enough to where the case wouldn't fit into the AI FL sizer. Also, factory necks can be real generous. If the reamer neck is smaller than factory - which is likely - and cuts only 1/2 of a new neck, you wind up with a step-down. I saw that on someone else's gun. Unless you know the exact measurements of reamer and existing chamber and they'll work together, it's a good idea to cut a whole new neck. Giving it a complete new throat is always a good idea anyway, you can choose your freebore. The thing about .004" crush is that it sounds ok in theory. In reality, brass headspace can vary by more than that from one case to another within the same headstamp. And longest-shortest among different headstamps I've measured variation more that twice that. There's no rule about this, the amount of crush can be whatever you want it to be. A new case touches the chamber only at the neck/shoulder junction radius. Closing the bolt you'll feel very little difference between .005" and .012" crush.

If the barrel shoots well as-is, it'll shoot the same as an AI only faster.

Ackman is right about the variation in HS dimensions in factory cartridges but P.O. Ackley designed his rimless AI chambers with a .004" smaller HS dimension than the parent cartridge.
Some folks cut their AI chambers without setting the barrel back at all and some people cut theirs with alot more than .004" crush. Either way will likely work but neither is a true AI as intended or designed by Parker himself.

I did some rifle work for P.O.Ackley's Custom Rifle shop back in the early 1980's and with regards to setting up headspace, it was held short to provide firm support of the case while fireforming. The amount of headspace, or lack of it, was considered just right when chambering a factory round would leave a slightly visible ring at the neck shoulder junction. -.004 provided enough of a crush to hold the brass without destroying it, and using the -.004 figure made it easy to repeat.

Rechambering to the AI is a ticklish operation mostly because of setting the barrel back and the shorter headspace dimension for the Ackley.Also you must consider getting the barrel markings turned correctly after setting the barrel back a turn.
Doing an H&H mag.is not so simple without setting the barrel back,lets say you have a long headspace dimension on the factory chamber.Doing it correctly would require setting the barrel back, also would have to contend with a correctly positioned extractor cut if doing an M70 CRF.
It all can be done,just hope the guy doing the job isn't hungover.I like the Ackleys because they look so good.

If you want it done right contact Ray Montgomery in Grand Junction,CO. powdr

In 2002 I did not know that and I chambered to fit standard brass without .004" interference.

The result was that if I try to load 257Robers ammo and fire form it into 257 Roberts Ackley Improved, there are problems. The firing pin pushes the case forward, the brass crushes .004" at the sharp edged base of the neck, and then the case gets stretched .004", causing case separation ~.1" behind the shoulder.
My 257 Roberts Ackley Improved VZ24 with Lothar Walther 2300 taper barrel will not form brass easily.
But it has shot .45" (5) shot group at 100 meters and I did not want to take it apart.
So I experimented until I found a way.

I lube up the case with 50,000 psi lubricant. That can be Moly-D, or many other lubes.
I put 10 gr of any pistol powder in the case.
I fill the rest of the case with Cream of Wheat.
I fire the case without a bullet.
This half forms the Ackley shoulder.
The next loading is full power and full accuracy.
After that, the shoulder is fully formed and there is no damaging stretching.

It took many experiments to come up with that process.
The same reamer has been used on 3 other rifles since, but those were shorter chambered.

My preference is to only do the work on a barrel that you can set far enough back to cut a completely new neck and throat. Factory chambers can be on the slightly generous side, and you can end up with a dual dia neck, so having enough metal to put in a fresh neck and throat is the best way to approach the job. A competent smith will have no problem with the re-chamber.

If it ain't broke then don't try and fix it..

A half inch gun is something one should leave alone..With the advent of the chronograph the Ackley Improved cartridges have died on the vine, you gain about 100 FPS in most cases..Even Ackley himself said the only real successes were the 257 Robts and 7x57 that apparantly gained 300 FPS in his guns. The gained more like 200 FPS in mine and I found that I could duplicate his goals in the 7x57 with a 06 box and a long throat without cutting my existing chamber.

Just my opine.