Home Forums Reloading Big Game Rifles Got dem ol' double groupin' blues

Have been working with a Tikka T3x Lite .22-250 with 1:8 twist, and it has a particular dislike for the bullet for which I specifically bought it, the Hornady 75 gr. ELD-M. Namely, it double groups them or wants to. Generally when a bullet double groups the answer is to seat it deeper. If groups are circular but spread out the bullet needs to be seated closer to the lands. Rule of thumb for sure but I've found it works far more often than not.

The background:
- Scope is an SWFA SS 10x42 in Warne mounts, screws torqued to manufacturer specs which are 25 inch/pounds - I used 25 on the bottom and about 22 on top to be sure not to stress the scope tube. Screws are tight and the scope isn't moving. No other bullet double groups so it's not the scope.
- Action torqued into the stock per Tikka's spec - 44 inch/pounds front and back.
- Lapua cases sorted for neck concentricity, all 100 in the batch are at .001" or less
- Bullet runout is held to no more than .0015" on all rounds, a Lee collet die and Forster benchrest seater see to that.
- No particular pattern to the double groups, i.e. shots 1-2-3 here and 4-5 there, it will generally put 1-2 together and then 3-4-5 go back and forth in no particular order.
- Have tried two seating depths, .010" and .050" with all of the best loads. I tried a full .100" but groups went all to hell, plus the bullet is seated almost below the start of the ogive. .010" gives the best groups as can be seen, or I should say the best two groups.

Here is a series with the same three powder changes but seated at two depths. #2 and #3 in particular show the problem, two very nice groups spread apart. The same charges on the bottom row but seated at .050" show the groups spread out but still tending toward separate individual groups.



I removed the speed bump and fired these groups, the 75 ELD-M is the top two. Again, double grouping, albeit with a wider spread. Fwiw, the rifle rlikes the Hornady 75 BTHP which are the two bottom groups, you can see the exact same powder charges and same .010" seating depth producing respectable 5 shot groups with it.



Obvious solution is to go with the 75 BTHP, but that's not a loony enough answer, I'm hankering after the increased BC of that ELD-M.

My next step is to bed the action into the stock but I was hoping you folks could help me go over the litany of possible causes for double grouping in case there is something in front of me I'm overlooking here.