7mm Mag freebore question

[muleman]

Good morning to the forum!
Just started on my latest project - my brother's 7mag. It's a 20 year old Browning A-bolt. I was cleaning it, mounting a new scope, checking screw tightness and measuring the chamber for OAL cartridge length. I don't have that wonderful Hornaday gauge so I was using the method described in Nosler 6. Using 140gr Ballistic Tips my measurement to the lands was 3.395 (I took 6 measurements with the largest variance being .002). That's the longest gap I've ever measured between an actual length and the SAMMI spec (3.290). At what length do you suggest I start my cartridge length for working up my loads? I didn't have the clip for that rifle but I will be getting it today for measurement. I'll be using IMR4350, RL19, RL22 and Magpro to start and then work up a load for the160gr AccuBond.
Thanks,
Scott

 

[DrMike]

Scott,

It sounds as if you have some excellent measurements, exercising great care in obtaining your data. Verify that the magazine will receive the cartridge. Browning magazines tend to be tight with amunition loaded to SAAMI specs. I'll suggest that you will not be able to load anywhere near the lands as result. Consequently, load to the magazine and adjust your charge to vary incrementally the harmonics of the barrel. You may be pleasantly surprised. I had a Browning Eurobolt in 7mm RM. It would shoot ten 162 grain SSTs into 0.90 inches. Recently, I loaded a 270 WSM that had 0.250 inches of freebore that gave me 0.25 inch groups with Swift Sciroccos. With that amount of freebore, one thing wish to try is to use 150 grain E-Tips which need a good jump to the lands in the first place. RL22 should give you some excellent results, especially with the 160 grain AB. Let us know how that works out.

 

[Polaris]

Being a C&R military rifle enthusiast, I am no stranger to excessive freebore. If your magazines will accomodate, you can exceed listed COLs to reduce your freebore. If not, you will be stuck with your max. magazine length as your max. COL. That shouldn't be a problem with a hunting rifle. Realistically, if you can acheive 1.5 MOA, that will be more than satisfactory for hunting purposes. That shouldn't be a problem in your case with a little experimentation. Some rifles shoot extremely well even with much more freebore than your rifle has. I once had an israeli K98 with more than a caliber of freebore when loaded to magazine length which was already much greater than listed max COL for 7.62 NATO. Even with the very crude K98 sights it would group aroound 1.5 MOA with 180 gr. Rem Core lokts.

A general rule I've found with more freebore in a rifle is to use faster listed powders with flat based, heavy for caliber bullets. Seems to give more consistent velocity and grouping. You may also wish to pay particular attention to your case neck thickness and consider a Lee Factory crimp die to increase consistency from primer ignition to rifling engagement. In your case, a 160 or 175 Partition or Hor 175 SP would likely be a good starting point. IMR 4350 should be a good powder for you. Good luck.

 

[Elkman]

My 7mm Mag shoots very well with 4350 and 162 gr Hornady BTSP bullets. As Dr. Mike says magazine length will limit your AOL.

 

[JD338]

I always like to start at .020" off the lands, providing the COL will fit the magazine.

JD338

 

[SJB358]

Same as Jim and Mike. I check to see where the bullet touches the lands, and load .020 off from there. If that is too much for the mag, I go to mag length. I am a hunter, so MOA is very acceptable to me. Scott, I imagine you will get plenty of accurate loads out of the rifle. Same as Mike, my 270WSM (with all bullets) is loaded to standard SAAMI specs and I have never messed with it, shoots clusters easily and I can keep them out to my max range. No IDEA where or how much freebore I have. Scotty

 

[phutch30]

I have a a-bolt thats 15 years old and it also has a long bore, so probably normal

 

[Antelope_Sniper]

suggest that you will not be able to load anywhere near the lands as result. Consequently, load to the magazine and adjust your charge to vary incrementally the harmonics of the barrel. You may be pleasantly surprised.

There's your answer.

 

[roysclockgun]

I was using a Browning B78 (falling block) so mag clearance was not an issue. For a number of years, may favorite bullet was the Nosler 140AB loading in my 7mmRemMag to an OAL of 3.364". I was pushing the bullet with 70gr. of IMR7828, producing an average MV at 10ft. from the muzzle, of 3290fps. This was a fantastic load for me with my longest field shot being on a mule deer at 404 yards. I knew at that range that I did not have to hold more than a few inches above the heart/lung area to kill the deer, as I took him across a canyon, resting on a handy flat rock that happened to be right next to a fence post.
If your best accuracy comes from an OAL longer than your mag, why not load up the long cartridges and hand feed them as your first shot? The balance of the full mag will, of course will be loaded with shorter cartridges that will feed through the mag. I believe that in nearly all cases, where accuracy is important because of range, you will likely not need a follow-up shot anyway. I have used this idea in my bolt rifles and I color code the longer cartridges with fiber tipped pen marked on their base, so that they are easy to recognize in their own separate box. Have a number of those "First Shot" cartridges in one box, in your gear and the mag fed cartridges in another box.
Maybe I am too picky, but I want to wring all the accuracy that I can get from my rifle, as I need all the help that I can get!
Best,
Steven

 

[muleman]

Thanks for all the input and advice! I picked up the clip for the 7mag yesterday and it measures 3.390 for an interior measurement. It looks like I will be able to go long with the bullets. Range report coming soon.
Thanks,
Scott

 

[JD338]

Thanks for all the input and advice! I picked up the clip for the 7mag yesterday and it measures 3.390 for an interior measurement. It looks like I will be able to go long with the bullets. Range report coming soon.
Thanks,
Scott

Excellent. If the magazine will allow you to get close to the lands, start out at .020".

JD338