I know that once you sight in a rifle or shotgun with a specific brand of ammunition and bullet you very likely will have a different point of impact when you switch manufacturers even with the same bullet weight. My question is a little more subtle.

In the main area that I hunt deer here in VA you cannot use center fire rifles...only shotguns and muzzleloaders. I use a ProHunter and use both the MZL and 2o gauge barrels.

So here is the question. I found that the gun loved the Federal Premium 20 gauge 3" magnum 275 gr Barnes Tipped expanders at 1900 fps. I mean on a good day 1 inch groups at 100 yards. As is always the case, Federal stopped making them a few years ago and my supply was running out so I tried the Federal Trophy Coppers...same weight and same velocity (275 gr and 1900 fps). Bought a few boxes and ran them through the gun....shot about 10" low and grouping were like 4". Pretty disappointing with the group size and realy surprised in the significant drop in the point of impact since Federal "claims" these are a replacement for the Barnes and are suppose to shoot almost the same.

Being resourceful I called Barnes to see who was loading that bullet for a shotgun. They told me about a custom ammo shop in New Jersey. I ordered a half dozen boxes of their 20 gauge 3" magnums loaded with 275 gr Barnes Expanders (they did not load the tipped Expander) pushing 1875 fps. I tried these out today and they shoot about 5" low, but grouped pretty good except for one flyer which I will blame on me. So grouping (my main concern) is better than the Trophy Copper and these slugs are probably what I will use once my original Barnes are used up.

So my question, what do you think is the biggest contributor to the change in impact? For the Federal Copper I assume they used the same powder, same shotgun case & primer, same sabot and the only variation is the bullets (which by the way look identical except one is Barnes and one is Federal). For the NJ slugs I can see a lot of possible causes...different sabot, different powder, etc.

So real question is "What is the most likely reason for the lower point of impact given bullet weights are the same and velocities are comparable? Just wondering out of curiosity.

This should explain it

http://cafehayek.com/wp-content/uploads/2014/03/miracle_cartoon.jpg

Sounds like to me the BC of the new bullets would be the varible in both your equations. They are lower because they are not flying as efficient as the original.

That is making two big assumptions...the biggest being they all really do weigh the same and secondly they are all going the same speed.

lol

My guess is that the barrel vibration and whip is different with this newer ammo you've tried. Velocity looks close, but is that measured velocity or published velocity? There could be considerable disparity there. But even if the velocities were identical, there could still be differences because of the powder used and therefore differences in the pressure curve, leading to the different vibration and whip of the barrel.

The Custom ammo from NJ was measured by them..... the Federal velocities were from the data on the box (but Federal is typically pretty close. On the original ammo (Federal with Barnes tipped Expanders) the bullet drop at 150 yds for a 100 yd sight in was dead on vs the data on the box.

Like I said. I was expecting a different point ..just wondering what the most likely contributors were.

This.

Thanks guys.....