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7mm mag powder choices

25 Jul 2011
@ 10:57 pm (GMT)

james hoskins

Hi I have been working up a 7mm mag 162gr amax load and have been using 2225, I see you suggest 2217 and 2213sc in another thread, I am currently achieving around .7moa accurracy which I want to improve on, should I be looking to move towards one of these 2 powders, is 2225 not the best choice?

Replies

25 Jul 2011
@ 11:03 pm (GMT)

james hoskins

Re: 7mm mag powder choices
I should have mentioned rifle is a 24" t3, 73gr 2225 is giving ~3030fps, I have also noted quite an increase in velocity once barrel temperature rises
26 Jul 2011
@ 09:23 am (GMT)

Nathan Foster

Re: 7mm mag powder choices
Hi James, I tend to use 2217 (H1000) as my go-to powder for the 7mmRM however, 2213sc (H4813sc) can work well in shorter barreled rifles. In your rifle, you are achieving very high velocities so I would not suggest a change just yet.

Your goal is not simply acceptable accuracy, it is extreme rifle accuracy.

The primary elements for this are:

Good bore
Good platform (bedding etc)
Good, basic, sound hand loads (doesn't have to be fancy)
Good shooting technique

It sounds like you have a very good bore to get this far. The increases you are seeing in velocity are most likely associated with the bore tolerences tightening up as the ultra thin barrel expands when heating. You can test this versus whether it is a powder temp sensitivity issue by placing a cartridge in the mag, lining up through the chrony screens, then cycling and firing the cartridge quickly before the brass has time to heat up. Chances are, you will find that its the bore tightening up, increasing pressure and therefore velocity (heavy copper fouling does the same). Suppresed T3 barrels suffer the worst, raising temperatures and velocities exponentially if the barrel is not left to fully cool between shots, also causing carbon caking at the case neck area of the chamber which gradually increases pressures further. Regarding the bore as an issue, make sure it is allowed to fully cool between shots.

If the rifle is unbedded, this could be an issue. Often, the T3 does benefit from bedding, though you need to go through the next lot of checks first.

The T3 is a very light rifle, generating a great deal of recoil inertia in 7mmRM. It requires extremely good technique to get the last little bit of accuracy out of such a rifle design. A bipod will induce vibration and open up groups so if you are using a bipod, it needs to go. Good forehand control of the stock forend, good sling usage and tension, a stable triangle are key (please read the 'Hold that forend' article in the knowledge base) Trigger let off weight needs to be neither too light (no good if the weather is cold and you can't feel the trigger) or too heavy. About 1.5lb is right for extreme precision.

No amount of rifle add ons will make up for a slight error of technique or poor technique, doesn't matter whether its a 15lb rifle or an 8lb rifle. I cannot re-iterate this enough, the trend towards shopping for kit to make up for slight through to large errors in technique causes a lot of disappointment in the shooting sports. Again, we are talking about taking the rifle from .7" to .5" and less. The T3 is incredibly sensitive to simple changes in sling tension with groups varying from .5" to 1" with the slightest changes. Learn to hold the rifle firmly, but without muscling the rifle, this is very subtle but is of the utmost importance in your endevours.

Changes in powder brand and seating depth work very well on finnicky bores. Stresses within the barrel caused during button rifling can create a situation where every reloading trick in the book has to be used to gain extreme accuracy. The T3 rifles are definitely prone to stress problems due to the thin walled barrels but it probably is not happening in your case. If it was a problem, we would be discussing double groups or how to get the rifle down from 3" groups at 100 yards. Start at 1mm/40 thou jump and test loads. If you have ticked all of your technique and bedding/sound platform boxes and the rifle still won't shoot, try a close jump, around .2mm or 8 thou. Changing to a faster powder will increase whip in a light barrel however it can sometimes produce a desirable vibrational pattern.

26 Jul 2011
@ 11:18 am (GMT)

james hoskins

Re: 7mm mag powder choices
thanks for that info
I have 2 other t3's 223 and 7mm08 which both shoot very well (.2" for 223 and .4 for 7mm08) I can get the rifle set to sit naturally on target and am wondering if there are adjustments I need to make to absorb the higher recoil of the 7mm mag more uniformly. I havent been applying any pressure to the forend so will try this on my next trip to the range.
What accuracy tolerances do you aim for to shoot ~700m
again thanks for the info, its very much appreciated, if I cant get things improved through technique and bullet jump I might be sending it to you for some bedding attention
26 Jul 2011
@ 09:50 pm (GMT)

Nathan Foster

Re: 7mm mag powder choices
No worries. For 700 meters, I like to be able to shoot .5" under field conditions (over my day bag). When a rifle shoots .7", I usually limit shots to 700 yards (637 meters), preferring to stick within 650 yards.

I do like to get client rifles down to .3" over the sand bags, .5" off a day bag but regardless of what I hope for, anything between .3 and .5 off the sand bags is optimum.

Good luck, please let me know how you get on.

05 Aug 2011
@ 09:04 pm (GMT)

james hoskins

Re: 7mm mag powder choices
I went to the range the other night as the frost was settling, I loaded a batch of rounds with less jump and uesd the sling wrapped as you outlined in your article, it took a while to get comfortable with it and 'find my spot" but once I had it sorted felt locked in solid to the rifle. Fired one shot off cold barrel then a 3 shot group, fouler was 1/2 an inch right and the 3 shot group had an external diameter of 13mm. Hope it wasn't a fluke, will load some more and do more testing, not sure whether the improvement can be contributed solely to oal or technique as changed both so will load more at previous length also. Being so cold I was getting bad heat shimmering off the suppressor after the group so couldnt continue.
Thanks for the advice, something certainly seems to have done the trick (a few more groups will confirm it)
06 Aug 2011
@ 01:39 pm (GMT)

Nathan Foster

Re: 7mm mag powder choices
Thanks James, I appreciate the update. Yes, its a bit frustrating when we change two variables at once. I am glad that you have the 'need to know why' attitude- even though the results at the range were good.

Just wondering, could you please tell me the max COAL versus the magazine internal COAL so we can discuss bullet jump a bit more.
15 Aug 2011
@ 03:55 pm (GMT)

james hoskins

Re: 7mm mag powder choices
SOrry about the slow response.
magazine length is 3.372"
current loaded round length (measured to tip) is 3.404
This means it is a single feed load.
I can load to magazine length of .7" with same poi so can use this for general hunting and single feed the long ones for the precise shots I need.
I would have liked to have to the accuracy from mag length but can live with this
15 Aug 2011
@ 04:58 pm (GMT)

Nathan Foster

Re: 7mm mag powder choices
Thanks James, much appreciated. Sounds like you have a good set up/ compromise regarding the dual loads. It is a pity about the mag length, can sometimes also be frustrating for .300 Win Mag users. However, this type of freebore is still within the realms of what can normally be expected from cartridges like the .308 Win.
16 Aug 2011
@ 08:37 pm (GMT)

Ross Goldsack

Re: 7mm mag powder choices
James,
you may be able to modify the T3 magazine and bolt stop so you can run your longer rounds all the time. I haven't seen a long action mag but I have modified my .308 mag by carefully cutting and filing the rear of the mag out, in my case I replaced it with a plastic block. I can post photo's if you require.
16 Aug 2011
@ 08:54 pm (GMT)

Nathan Foster

Re: 7mm mag powder choices
Unfortunately they are the same magazine/floor plate assembly as the .308. The magnums are simply an unblocked version of the .308 mag you have. The one I have here measures 85.7mm internally. It will feed cartridges smoothly at a COAL of 85.2mm for a .5mm gap. Trouble is, most 7mm rem Mags call for a COAL of around 86.5mm (max COAL 87.5mm) with the A-Max. The .300 Win mag is much longer again. I had popped the mag length question to James in the hope that maybe there are variations in models that I have missed (hoping there might be long mag versions floating around somewhere), but alas, they are all the same.
15 Oct 2011
@ 11:14 pm (GMT)

james hoskins

Re: 7mm mag powder choices
Just a follow up
Have been away from this rifle for a while. I though I had the Amaxes sorted but they were proving to behave inconsistantly typically printing 1 shot high 2 shots touching an inch and a half below. I tried a heap of different seating depths but ended up swapping to 168gr berger vlds to see how they would go. 9 rounds later had a load producing nice uniform groups with the best measuring .48" to outside edges.
120 thou jump
fed brass
cci primer
2225 powder
yet to chrony but amaxes with sam amount of powder were yielding 3030 fps
one real perk is that this load easily fits into the mag. I was really hoping to get the Amaxes working due to their rep in terms of terminal ballistics but hopefully the bergers will perform well enough on this front
21 Oct 2011
@ 11:49 am (GMT)

Nathan Foster

Re: 7mm mag powder choices
Thanks James. If you get a chance, have a play with blow torch annealing the VLD as outlined in the knowledge base. Make sure you hit them plenty hard with the heat. Following on from this, be sure to keep autopsy records and study wounding closely. If you can get your annealing right, you will be just fine.
 

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