Be sure. Back off around 10%. What I do is load a couple at the reduced loading, fire them and look at the case for pressure, etc. Then, if everything looks and feels okay, I go up a grain. And so forth.
If you're not "pushing the evelope", perhaps just loading a couple of rounds with the same charge might give you your answer. That would be something for you to decide and would, in my opinion, depend on your powder charge relative to max.

I'm looking for any info about how much a powder charge should be lightened when switching from regular to magnum primers and keeping all other factors constant. I've heard that .5 to 1.5 grains less powder is generally acceptable.

Any comments or info is appreciated.

Steve

I suggest that you look in your reloading manual they say what primers to use. Shrapnel is not fun.

I used to use magnum primers for ball powders only. But several friends have convinced me that they are very useful for all powders here in the very cold climate.

That's why I'm trying to extrapolate reduced powder charges to provide for the increased pressure from mag primers.

I haven't found anything in my manuals that address this. So I thought I'd see if anyone has some experience with this.

I'm not "going for broke" with my charges. I like to stay several grains under max(2-4) as this is where I usually get my best accuracy.

Steve

you don't say handgun or rifle. I loaded some 41 mag ammo with 8.5 grains of Unique and a 215 grain swc with both CCI-300 & CCI-350 primers. Unique is a powder that by itself doesn't require magnum primers. after I got done with the chronograph there wasn't any velocity difference. the ammo with the CCI-350 primer while having about the same average , the extreme spread was cut in half. I now load all of my 41 mag ammo with CCI-350 primers so I don't have to keep anything separate if I need to use a much heavier load that uses H-110 or WW-680. so just because you are using a reduced load doesn't mean that you have to switch from a magnum primer to a standard primer. try both and maybe the magnum primer will shoot better.

I have tested magnum and regular primers with H4831, H4350, and Varget. I have found no change in pressure or muzzle velocity that cannot be fully explained by normal, random variation. There may be combinatins where the primer does make a difference in pressure, but I have not yet found it.

I should have mentioned that I'm reloading for my rifle(7mm-08 T/C Encore, 24" barrel, 140 grain Partition with Varget or RL-15).

Steve

As a last step in load development I usually try as many primers as I can to see what difference, if any, they make to both accuracy and velocity. In my experience it is rare that there is much difference in velocity. I would have said NEVER but just last week I substituted some Federal 210M's where I had been using WLRM's and CCI250's. I got a consistent 60fps MORE with the 210M's than the other two magnum primers. That's the greatest velocity difference I have ever seen with primer substitutions.

In most instances where I've substituted primers the difference in velocity is minimal. It's there, but it's not much. I do know that many who have tested the substitution with some pressure trace systems have reported some significant rises in pressure which brings all of us back to the admonition that we reduce charges when substituting components. While I'm a bit more wary of substitutions, unless I'm at the upper limits of safe pressures I usually just substitute freely. I've not had a problem yet. Not even a sticky bolt.

As for substituting magnum primers because of extreme cold, I used to do that too. I hunt in your state as well as Wyoming, mostly in late season hunts. I've used standard primers consistently the last 10 years or so and have hunted in below zero weather frequently and haven't had ANY ignition problems. I don't think I would substitute primers solely for that reason.

It must be the "338" thing but I have had the same exp. as Bob338. After I find a good load I'll swap primers, match, mag, diff. brands. Sometimes accuracy/consistency improves, sometimes not. The only time I have had pressure problems is if I am running loads right at max. 1gr reduction & work back up would seem prudent when switching to a mag. primer.

Denton,



Funny I found basically the same thing. Actually, in the 30-06 I was testing, IIRC there may have been a trend to slightly lower muzzle velocities with the magnum primer, all other things being equal.



Damn, now I am going to have to go look that stuff up!



I found some of it!



30-06

Winchester cases (internal volume range - 69.1 to 70.7 grains of wate)

180 grain Sierra BT (GameKing)

54 grains of IMR4350

CCI200 primer

2581 fps

2684 fps

2594 fps

2643 fps



CCI250 primer

2709 fps

2724 fps

2736 fps

2694 fps



So for those the magnum primer did increase the velocity and pressure.



30-06

Winchester cases (internal volume range - 69.1 to 70.7 grains of wate)

180 grain Sierra BT (GameKing)

55.0 grains of IMR4350

CCI200 primer

2692 fps

2710 fps

2712 fps

2704 fps



CCI250 primer

2672 fps

2686 fps

2666 fps

2674 fps



Interestingly there is lower velocity with more powder and the CCI 250 primer than we had with the 54 grain charge previously! There is also very little difference in muzzle velocity between the two primers. What difference there is makes the magnum primer slower!



30-06

Winchester cases (internal volume range - 69.1 to 70.7 grains of wate)

180 grain Sierra BT (GameKing)

56 grains of IMR4350

CCI200 primer

2743 fps

2757 fps

2740 fps

2778 fps



CCI250 primer

2754 fps

2729 fps

2783 fps

2769 fps



Pretty much a dead heat there.



I still haven't figured out what was going on with that 54 grain load and the CCI250 primer.



These loads were all shot in the same rifle, which had a 22" barrel.



Now for a little more information.



S&B brass (case volumes from 71.2 to 71.8 grains of water.

CCI 250 primer

180 grains Sierra BT (GameKing)

22" barrel

55.5 grains of IMR4350

2742

2720

2738

2744

2772

2749

2772

2778

2775

2749



S&B brass (case volumes from 71.2 to 71.8 grains of water.

CCI 200 primer

180 grains Sierra BT (GameKing)

22" barrel

56.5 grains of IMR4350 ( MAXIMUM LOAD < !--color--> )

2756

2823

2840

2816

2807

2807

2796

2785

2825

2792



The 55.5 grain load was not as accurate as the 56.5 grain load. It turned out this particular rifle did not like Boat tailed bullets (none of these loads was very good) go figure.



Anyway, there is ALL the data I am going to dig up. You guys be the judge.



I would recommend that you reduce YOUR non-magnum primer load by at least 5% and work back up with the magnum primer, just to be prudent (read safe). < !--color-->

ASS_CLOWN