No I have not tried Speer 100 BTSP. I shoot a 6mm Rem with Hornady 100 BTSP & have shoot my 2 smallest groups 7/8 inch at 300 yds yes 300 yds nice groups. I will say I have only done this twice Remington 700 BDL 6mm factory barrel,made about 1974. Scope Steel tube 3x9 40mm Weaver 35 year old scope, It has been glass bedded, & trigger lighten. In reality for 3 shot groups it averages 2.1 inch groups on average when shooting at 300 yds. Look up on Hornadys web site .243 100gr BTSP has a picture of a Mule Deer I shot a couple years ago.