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7mm options

26 Dec 2010
@ 07:20 pm (GMT)

james hoskins

Hi I have been looking to build a new rifle, the aim was to have something capable of pushing a 162gr amax in the 2900fps range, have it suppressed and not excessively heavy to have it as a reasonable carry around long range capable setup. I was looking at 284 and 280/280ai but have heard the 280 would be the only one to feed reliably. The other thought I have had would be to buy a 7mm mag and suppress that with some milder loads but that would require a heavyier suppressor. I see you have a left handed t3 on trade me at the moment, how would a 7mm mag go if cut back to 22"? I had envisaged a 280/284 with around a 24" barrel but think 2900fps might be a big ask for this type of setup, have you worked on a similar type project?

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27 Dec 2010
@ 12:09 am (GMT)

Nathan Foster

Re: 7mm options
Hi James, sounds like you have a lot going on.

Ok, first thing is to define (to yourself) long range. If its say 700 yards as a typical maximum and 800 yards as absolute maximum then your goal velocity of 2900fps will be perfect. If you want to shoot longer, its better for the sake of the animal, if you have more power. Forget about people who can shoot the eye out of a magpie at 1200 yards with a .280, we all know the .280 is accurate and capable of making hits out long, what I am talking about is wide wounding at long range along with low wind drift. So thats the first thing you need to address.

If you want a light carry rig, why do want a suppressor? Why not have a 26" barrel and no suppressor. The rifle will be relatively light and in .280, will achieve 2900fps. As for the .280AI, of the rifles I have worked on for clients, I often find that they have been loading beyond acceptable pressures, limiting case life to 2-3 reloads for 2980fps. From a 26" barrel, 2900fps should be the goal velocity. The AI allows for more positive headspacing along with less need for trimming, these are its strengths.

Down loading a 7mm rem mag may not work, many rifles do not acheive optimum accuracy until they are at full pressure. Its simply too much of a gamble. My wife's 7mm RM has a sweet spot at 3120fps (24" barrel) with the Amax. I tricked up another 7mm RM a few weeks back, 26" barrel, sweet spot was 3125fps. Accuracy was only average at 2925-2950fps in both rifles however, I suppose further experimentation could produce desirable results, I really can't say for sure.

As for barrel length, in the 7mm's, from 22 to 28", each inch of barrel will give or remove around 35fps. I other words, a 24" barreled .280 will give 2825fps. I have found that some will give optimum accuracy at around 2850fps but I have not been able to go higher when looking for optimum accuracy.

You really do have a lot to consider. If for example, you opted for a 7mm Rem mag with a light contour 24" barrel and suppressor, you would have a relatively mild recoiling long range rifle of moderate weight. As I have said, first you need to establish what ranges you intend to hunt out to.

Steph's rifle is a left handed Montana M1999 with a light weight 24" TF barrel on it, no suppressor. The current pet load is the Amax at 3120fps. Rifle weight is 10lb and Steph hauls that rig everywhere, doesn't matter how big the hill is. She shoots .330 off sand bags and like me, shoots .5" exactly off a day bag. She has never liked bipods so that is a good weight and bulk reduction. Recoil is moderate but like all magnums, you have to get yourself in the 'zone' to shoot accurately out long, all the stuff I have mentioned in the hold that forend article.

Sorry, I don't have a T3 rifle on Trademe, doesn't sound familiar. I steer clear of the T3 for project rifles due to the limited magazine length (in the long actions) and lack of a decent recoil lug. The T3 is more of a good utility rifle, light weight and accurate if you can manage the recoil due to the said light weight. I do have to work on them throughout the year, basic tune up jobs, most are acceptably accurate before they arrive, shoot only slightly better when tricked up, a few are bit funny in the bedding, had one with a flawed barrel that had to be thrown out last year but no blow ups as happened in the U.S.

Hope that helps for a start.
Nathan.
27 Dec 2010
@ 09:54 pm (GMT)

james H

Re: 7mm options
that is exactly the info I wanted, i will have a think about things to try and decide what route to go, after posting the question I had moved to the thinking of the 26" .280. I had in mind 700 as typical max and 800 as outright max when coming up with the 2900fps goal.
I will think more about wether to go the 26" 280 or 24" 7mm mag. Would a remmingto 700 action be a good place to start?
27 Dec 2010
@ 11:53 pm (GMT)

Nathan Foster

Re: 7mm options
Yes, the Rem M700 is the pick of the bunch, primarily because of its simplicity (easy to get accurate), the long magazine box and the the availability of aftermarket parts and mods in New Zealand. If you are left handed, an old Sako AV or L series is also a good base for a custom build. Steph's Montana is a good action but I don't know if these are still coming into the country. Savage is another option.

You will definitely obtain wide wounding at 700 yards with the .280. On days where the wind is low enough, you will be able to stretch shots out to 1000 yards and obtain wide wounding with solid chest shots. When the winds are up a bit and you run the risk of liver shots, I would try and stick to the 700 yards as a safe limit and 800 yards as maximum (7mph winds gusting up to say 12mph). If you shoot a lot, you will find that my suggestions are pretty 'safe' and that you will be able to achieve excellent results. Practice is the key.

One more thought, if you do go for a light contour barrel (say Number 2 contour), the True-Flite has a nice long knox if you opt for their standard knox (its chunky at the receiver end) enabling you to get very good 3 shot groups without wandering due to heat. After 3 shots, groups will open right up with this contour. It just pays to know what to expect from a particular contour. A number 3 contour will still get hot and wander a little on the 4th and 5th shots but not quite as much as the No.2 contour. As can be expected, as you approach a full No.5 contour, 5 shots groups produce very good accuracy but the barrels are very heavy, not suitable for what you are wanting. Besides, its just as easy to burn out the throat and ruin a No.5 when firing multiple shots in quick succession.

Don't muck around with light contour fluted barrel, the Finn-Lite's are set up this way (No.2) and are very finnicky, have to be left to cool between each shot. Not good for follow up shots at long range.
12 Mar 2011
@ 04:58 pm (GMT)

Jim Moseley

Re: 7mm options
What load does your wife shoot to get a 162 a-max up to 3120 mv? My 7mag , Hart 26", 9 twist, #6 contour, HS Precision stock, jewell trigger. My load is 72 grs of H-1000, GM215M primer, Win brass, 162 A-max seated .007 off the lands, MV 2980 fps. Best group is 2" at 820 yds. Love to keep the accuracy but would like a little more MV. What's your secret to the speed??
Thanks!
12 Mar 2011
@ 05:00 pm (GMT)

Jim Moseley

Re: 7mm options
Forgot to include the barrel length of 26 inches.
12 Mar 2011
@ 05:23 pm (GMT)

Nathan Foster

Re: 7mm options
I just looked at Steph's data and the above is wrong. Muzzle velocity is 3093fps with an ES of 14fps. Load is 69.5gr H4831sc (2213sc) with the Amax and 40 thou jump. Sorry about the incorrect MV.

I think the key to Steph's speeds in the 24" barrel are the 4 groove canted land barrel design from True-Flite. Its a very generous design regarding velocity.

I usually get 3125fps out of 26" barreled rifles (A-Max) using H1000, starting at 71 grains and carefully working upwards.

The 7mm Rem mag seems to be stock fodder on my bench these days, I have been working on one for a client today. This one is a 23" barreled M700 SS Laminate (all the ones in NZ have 23" barrels- why I don't know, am guessing the U.S ones are the same). Have only started preliminary load development using the clients components which include H1000 (ADI 2217), the 168gr VLD (for Red deer at long range), win brass and Fed 215 primers. I have tricked the rifle up and by shear fluke, the start load is very accurate. 71 grains H1000 is producing 2900fps, .320" and a very low ES. I will now work up to 72.5 grains and study accuracy and pressure etc. This was no easy project, the bore had to be given some serious punishment to remove fine burrs re my break in article. That bore was given two choices, either conform to a violent break in or end up as scrap. Prior to the beating I gave it, average group size was 1.2" and fouling was severe.

I am now noticing a .2mm (around 10 thou) variation in ogive/ max COAL from VLD to VLD. Will most definitely be sticking to 1mm (40 thou) jump for all future testing/ load work. This is not to be confused with the over all length of the VLD or differences in meplat. Am refering to differences in ogives.
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