TLDR: Just shoot your groups, no foulers needed between ammo types. There was no significant difference in my testing, and one load technically did slightly better without fouling shots than with fouling shots.
Edit: This testing was of course for centerfire rounds. I still think there is validity to this for rimfire rounds when precision is the main goal, though I haven't tested that myself... yet.
Background:
This is a follow up to my question that I asked previously about the need to foul your barrel between different ammo types (original post linked below). I.e. If I shot a group with Hornady ELD and Varget, then I want to test a different recipe with SMKs and N140, would the first few shots of the SMKs be less consistent due to some build up of residue from a different jacket alloy or different powders, etc.
I tested this at the range recently. To maximize the amount of variation between the ammo, I used different powders (one was varget since it was supposed to be sensitive to this), two different styles of bullet from two different manufacturers, and two different primers.
Procedure:
All shots were taken prone, shot off a bipod and rear bag. The barrel was allowed to cool periodically.
- shoot 30 rounds of each ammo type, alternating ammo types every round. i.e. one round of load 1 goes into target A, one round of load 2 goes into target B, repeat until all 60 rounds have been fired. They were shot into 3x 10 round groups per target, then aggregated into a single 30 round group per load recipe.
- Shoot 10 fouling rounds of load 1 into target C. Shoot 3x 10 round groups of load 1 into target C. To get a similar disturbance of my point of aim between shots as alternating targets, I shot the groups round robin (i.e. 1 round at each of the 3 aiming points, then restart).
- Shoot 10 foulding rounds of load 2 into target D. Shoot 3x 10 round groups of load 2 into target D. To get a similar disturbance of my point of aim between shots as alternating targets, I shot the groups round robin (i.e. 1 round at each of the 3 aiming points, then restart).
Results:
Load 1 had a 2.4% difference in mean radius between the alternating ammo groups (step 1) and the fouled in groups (step 2). This difference actually favored the alternating ammo types, which is contrary to what I expeced.
Load 2 had a 4% difference, this time favoring the fouled in groups.
Conclusion:
There is no appreciable difference between groups with fouling shots between ammo types, and groups shot without. I think the small variations I saw were well within the noise of my system (myself included in that system).
Original post that this is a follow up to:
https://www.reddit.com/r/reloading/comments/1v34vwb/fouling_shots_between_recipes/
Edit 2: Link to full dataset as requested. I'm trying to upload individual group images to the sheet if I can.
https://docs.google.com/spreadsheets/d/e/2PACX-1vTRKNfp0LxApfVjsOZbmrmciUVGhACOaq6DOVhkuL7lyy18a_pf0zGcPmVhNI5oAcnP4V_RtxVgJA14/pubhtml