.260 COL issue

[jgh4445]

I'm working up some loads for my .260. The max COL in the book is 2.800. The max that will fit in my detachable mag is 2.940 and it will easily chamber in my rifle. Is there any danger in exceeding the max listed COL? I'm guessing there is a max listing for a reason. Is it for the aaverage mag length or for safety reasons?

 

[Bart B.]

No danger if you work up loads correctly.

All max loads are established with the developers' objectives, conditions and standards. Most are only a guess; few have real pressure data. Pressure data they list is for their barrel, component lot numbers and system. It may be too hot for your barrel, components and your measuring method.

Start at minimum then work up in 1 grain increments.

 

[jgh4445]

Thanks. None of the loads are max. Its funny though, using Nosler Trophy Grade factory ammo, loaded to 2.761 ( shy of the 2.800 listed Max COL) it shoots a ragged one hole group. Thats with 130 gr Accubonds. I'd like to use 125 Partitions. At the 2.800 depth it shoots about a 1.5 inch group at 100. Not acceptable.
I noticed that I had a lot of room left in the mag so I seated this batch at the max for the mag. Thats 2.940. Playing with COL I discovered that this Sako must have as much freebore as a Weatherby. I can chamber a round just barely seated in the case, at a length of 3.065, without touching the rifling. 3.070 does touch and stays in the barrel when the case is ejected. I wonder should I have seated them the same as the Nosler factory stuff instead of making them longer. Totally different bullet profiles though. I guess only a trip or two to the range will tell.

 

[Bart B.]

I suggest seating bullets deep enough for safe handling and chambering. Doesn't matter much if they've got 1/100th inch or 1/10th inch jump to the rifling as long as the rounds are pretty straight. Most bullets can have a 1/10th inch spread in bullet jump to the rifling and accuracy is changed insignificantly across it. If you can shoot your stuff into no worse than 1/3 MOA at 100 yards, then some refining of bullet seating may be needed.

Also, I would shoot at least 10 shots per test group; 20 is better. A 5-shot test group's size means twenty groups of them will be from 53% bigger to 33% smaller. 20-shot groups indicate twenty groups of them will be 12% bigger to 11% smaller. For 10-shot groups, the numbers are 27% bigger to 19% smaller.

Here's my rules for shooting test groups for system accuracy. If several of them are not within 10% of the same size, they don't have enough shots in them to be meaningful. The system's accuracy is what the biggest group is. Smallest groups happen because one of two things made them tiny; every round was perfect and shot that way or all their variables cancelled each other out for the most part.

 

[jgh4445]

Interesting figures Bart. Thanks. I'll load some more for testing.

 

[jgh4445]

Went to the range with some loads. I used RL19, H4350 and 4064. I loaded 5 rounds of each seated at 2.761 (Nosler factory load COL) and 5 each seated at 2.94 ( my magazine max length). Everrything was the same with all loads except for the COL. I was surprised to find that the rifle seemed to prefer the shortest COL with all loads. The RL 19 shot a sub moa 5 shot group with the short COL and the long COL caused the bullet to impact 4 to five inches higher, with about a 2 inch spread. Same results with the other powders, much higher and a larger group. I didn't think the COL difference would have caused that dramatic a difference. I do believe the results would have been the same with a 10 or 20 shot group. Now, I'm going to tweak the best group trying different primers and a bit more powder in 2 tenths of a grain increments. Only one change at the time of course.

 

[Bart B.]

If anyone believes a 15 to 20 shot group with a given load will be the same size as a 5-shot group must also believe every roll of a pair of dice will produce the same number; always the same one some number from 2 to 12.

Every shot you make includes several variables about a zero point. Same as rolling several pairs of dice at once. A 7 with each pair is a zero for its variable, snake eyes is a -5 numerically up through +5 for box cars. A very normal statistical situation. What's the odds of having all pairs a 7?

Get two boxes of 22 long rifle then shoot twenty 5-shot groups at 100 yards. That helps explain why 5-shot bench rest groups at 100 are under .020" but many of those groups in aggregate groups average about .200" and their biggest single one is about .300". The .0077" 5-shot record holder has no aggregate records. That's the luckiest 5 shots I know of.

 

[jgh4445]

Not arguing with your math. Just stating that I think the shorter bullets would all strike at one height and the bullets seated longer would all strike at a higher elevation on the target. Now if you're saying that if I shot 20 of the longer rounds that somehow they would start impacting the target at a lower point, then, I think I'd have to question that.

 

[Bart B.]

Its very possible longer rounds (shallower seated bullets) will strike at a different point in elevation. Peak pressure and muzzle velocity will be high with bullets seated against the lands. Seated deeper a few hundredths to a tenth of an inch or more with a shorter COL, velocity and pressure will drop, Seating them more deeper will start increasing both velocity and pressure. How much varies with the load and the rifling angle.

Accuracy may well be the same with several seating depths of the bullet. You may or may not see a group center change at 100 yards. The bullets you mention from a .260 Rem will have a bit less than 2/10ths inch vertical shot stringing with a 100 fps spread in muzzle velocity; all else being equal. To see that much easily means you have to shoot your stuff into no worse than 1/4 inch at 100 yards to resolve that much shift in group center.

Therefore, I don't think its an issue at this time in your experiments. I think there's too many variables of unknown dimensions in your system (you + rifle + ammo) that separating those of the ammo from the other two will be difficult. Such is life with humans shooting their reloads in factory rifles.