First...I am doing a 510 AR right now...a .510 cal using a shortened to 2.65" Rigby case...Lapua and Rigby both have the same size rim and base...within 0.002". I'm using an older F111NS, 1.345" OD, 1.055" x 20 tpi action. It has been at various times a 25-06, 7mm RM, 30-06, 300 WM, 375 H&H, 416 Taylor and now(soon to be) a 500 AR...and the '06, WM barrels are still swappable along with the various box mags...the 375 H&H is on a newer Savage LA and the 416 Taylor barrel is now on an old Tang model Ruger M77.
Second...ANY action can be destroyed if you get stupid with the pressures...ANY action will flex...some more than others...ANY mauser action is flexible because of the construction...I keep seeing this cautionary statement "something's gonna happen IF..." scattered all over the net...it's like saying a car will kill you IF you get run over...stay off the street... or use your eyes...EVERYTHING in this sport is inherently dangerous, you just have to play it safe every chance you get...and keep your head when all others are loosing theirs. Besides in todays litigous atmosphere NO ONE is giving advice without all the wiggle words...
Third...it is a fairly simple, straight forward barrel/chamber job...DON'T USE A NUT...you get the full size of the barrel that way...before you jump on me, do some pressure calculations...you will see even with the smaller barrel OD and even with a nut there is plenty of strength as long as you stay below 65KPSI.
All that need be done is the following;
Have ANY 'smith or machine shop grind a magnum bolt head face rim out to 0.600" ID(leave a little wiggle room for the case head)...I did my with a lathe(1000rpm), air grinder(20Krpm) and 1/4" carbide safe end burr...it left a nice ring so the case won't slip off the bolthead.
I did all the other mods taking about a weeks time futzing around and also doing other projects...it shouldn't take a real machinist more than about two hours at the most to do all the other mods...INCLUDING installing the barrel.
Using a standard 300WM box mag...cut off the two small bottom bends to allow the spring to set on the bottom of the cutout...cut back the top notch 1/8"and re-contour - needed so the larger case will release right...bend the lips slightly so the case will feed - can be done with a pair of pliers, a small cresent wrench etc, and a small brass hammer. You are only changing the dimensions about 0.030'" per side.
You need to make a new follower if you want 3 down, otherwise the OEM mag follower works great for 2 down....3 rnd follower (ALUM) - 0.850" wide, 0 .300" thick, milled 0.080" deep on the bottom for the spring to seat, an 8-32 hole drilled and tapped for the spring screw, and a 0.110" deep groove slightly offset to the left and 0.590" wide for the case to sit in. YOu also need the box 2.00" deep from the top of the right frame rail plus 'Z' kink the mag box a bit about 3/4" from the bottom so it is 1" wide for the lowest round.
I've done all your work for you....
I have a end view of the mag box profile including the follower, but that will cost you MONEY...
McGowen is the goto barrel maker...$250 - $500 depending on how fancy you get...and a couple weeks wait...much faster than any other barrel maker I've used. My barrel, using a 1.5" blank to get the full 1.345" receiver OD, is 33", chambered, contoured #7, fitted and crowned will be $350-400, not sure which yet...but close...CM barrel alone is only $155, but they have the reamer so I will let them do the rest otherwise I would to all the work...33" bbl - add $45, contour - add $30, chamber - add $72, thread - add $60, fit - add$45. Interested...goto McGowen Precision Barrels Home
For those that want a full 3.60" mag, you will have to go to the 375H&H mag box and have your action milled to fit, plus a screw hold-on conversion added to the back of the box...if you're smart.
I can't comment on the Stevens...I've never worked with one, but I've done a bunch of Sav's LA and SA since the early '60's...also a bunch of Rem's and Rugers.
The REAL DEAL with ANY conversion or ANY rifle/cartridge is not being a DUMAZZ with the pressure...the rest is straight forward reloading samo-samo...YOU ALWAYS have to option download!!!
Been doing this wildcat "stuff" for a very long time...wish people would keep their faces out of the apple barrel when they don't have any teeth to bite with, but it is human nature to jaw flap and say nothing with a lot of words...even worse on the net.
I emailed one of the 'smiths mentioned in one of the threads for the cost of doing the bolt head and maybe some of the other work...but got no reply...guess he was too busy or didn't care...or???
To anyone wanting to do this conversion...be safe with the reloading...REMEMBER dropping the pressure 10% only looses 5% velocity, usually is 100% more accurate, 1000% more safe and you brass and rifle will last a whole lot longer.
not to mention keeping your hide OEM.
This is NOT a DGR rifle, but as long as you don't short stroke or mess up jacking a round it is as safe as any other push fee. It will make a humongous pig/elk/moose/brush/T-Rex whacker with 400-600 gr bullets...hard cast OR jacketed...and you DON'T have to load it to the gills to get the whackadoodledoo.
A 338 Lapua case is built a lot stonger than most other brands of brass which will give you a smaller case volume so if you use one be cognizant of this fact...otherwise just be cognizant of ALL the associated problems inherent in reloading.
Luck on your projects.