Help me figure this out

[Bob A]

I am working up a load for my Marlin 336 30-30 using Lyman 173 gr cast.
I am testing loads with both Reloader 7 and SR4759. Getting
pretty good results at around 14.5 gr of SR4759 and 20.0 gr Reloader 7. Which should put me at 1500-1600 fps.
Problem is that there seems to be 2 distinct points of impact. A typical 3 shot
group will have 2 touching and 1 shot down and right 1.0 - 1.5 inches.
Sometimes the first 2 touch. Sometimes 1 and 3 will touch.
I am cooling the barrel for 5 minutes between shots and it never gets very
warm.
Shooting 50 yards.
I am shooting off of sand bags under the receiver.
Using a Williams peep with the smallest hole (I don't remember the size, but its the smallest I could get from Midway)
Trigger is very good. I installed a Wild West Happy Trigger 2 years ago.
Bullets are sized to .311. Barrel slugged .309. Bullets sized .312 can be hard to chamber, so I think they are a good fit.

So, what do you think. Is it barrel heating (do I give more than 5 minutes?).
Is it technique?
Is it normal?

Bob A

[86er]

Maybe it is as simple as the gun shifting in the sandbag. Try picking up the whole rifle and setting it back down between shots. You could be having a slight shift at the second or third shot.

[86er]

Another thought - the powder is shifting in the cases due to recoil. Point the gun upward and tap the receiver for each round and see if it makes a difference.

[Chuck 100 yd]

Shoot those loads over a Chronograph ! Could be as simple as a change of primer or maybe 1/2 gr powder either way will settle her down. Do your gas checks fit good and tight? Could be loosing one once in a while?

[Bob A]

Thanks for the inputs.

I am loading one at a time. Picking up the rifle and loading the cartridge with the rifle pointing straight up. Then settled back down onto the sandbag.
Gas checks seem to be tight. I don't know if some are falling off. How would I tell?
I am using CCI LR primers and Win cases. Is there a better primer choice for this kind of load?
I think the only other options available locally are CCI Magnum, BR or Winchester LR.
I haven't shot them over the chrono but that's a good idea. Maybe I will set it up next week.
I try to take a mental health break on Wed mornings and sneak out to the public range. They open at 10:00 AM on Wednesday and its usually pretty empty till noon.
I can (and do)shoot at home on the weekends, but the range here is pretty nice and its a lot better than 2 hours in the office.

Bob A