newb question: shortened barrels and velocity

[welkin]

Recently I shortened the barrel on my 6.5 creedmoor t3x to 18". In another thread, I read that someone was getting about 2550 fps shooting the same factory round (eld-x) with their 18" barrel (on a bergara). The box of ammo says 2700 fps. I went out to re-zero at 100y. Previously my rifle had been shooting 2" low at 200y and 10" at 300. With the shortened barrel, i'm not at about 4" low at 200y and 14" at 300y. My confusion started when I looked at the table on the ammo box and saw these new drops were very close to what my rifle should have been doing before I had the barrel chopped. According to the Swarovski ballistic program (using because I have a z3 with the BRH reticle), with a 100y zero and 2700 fps, the 200y drop should be 3.84" and 300y should be 13.65". So before I shortened the barrel, I must have been getting significantly higher fps because according to what I observed while re-zeroing, I am now shooting at about 2700. I am living at sea level and it was probably around 50 degrees when I went out shooting. Maybe I am making a stupid mistake, but I really don't know what it could be. Any insight would be appreciated. Thanks

 

[eric1115]

Trying to true velocity using drops at 300 yards is really really tough since the drop is so little. Group size produces lots of noise and the signal is very weak at 300 yards. 600ish is the min I would be trying to true velocity using drops.

Edit: if you're set on trying to do this, definitely need large number of shots to do it. 10 shot (preferably 20) minimum group at 100 and 300 (and absolute dead nuts ranges) are necessary to find actual group centers.

Edit 2: that ammo can vary quite a bit from lot to lot, both POI and velocity. If you've changed lots of ammo, things could easily be different due to that switch.

Edit 3: magnet speed sporters are super accurate, and guys are almost giving them away now as they upgrade to Garmin chronographs. Probably almost cheaper than the ammo you'd need to get solid numbers from validating via shooting and measuring drops.

 

[HandgunHTR]

I would borrow (or better yet buy) a chronograph and determine what your actual velocity is.

 

[welkin]

I would borrow (or better yet buy) a chronograph and determine what your actual velocity is.

This is definitely my next move

 

[waspocrew]

I would borrow (or better yet buy) a chronograph and determine what your actual velocity is.

This. A chronograph will help take a lot of the confusion out of the equation.