Taller sights will increase MPBR?

Tex S · December 16, 2010, 09:01 PM

I have been using my new chrony and it seems to be working well. I finally got to test some loads I made for my 270 win. Here is the load...

Barnes 130gr Tipped TSX with a BC of .392
60gr of H4831 (careful, this is a max load in my Lyman manual)
Fed 210M primers
Average velocity is right at 3050 fps

So I was wanting to calculate the MPBR of this load. I went to the JBM website and started playing with their ballistic calculator. I think I have everyting entered correctly, and it looks like the MPBR for this particular load (with the sight being 1.5" off the bore and a 6" kill zone) is 296 yards, with a zero of 251 yards.

I have found that if I increase the sight height the MPBR increases a fair amount. This baffles me. What am I missing?

I also noticed that if I switch to the Barnes 110gr Tipped TSX bullet (BC .377) and keep every other field (including velocity) the same, I actually lose yardage on my MPBR. Is this due to the lower BC of the 110gr bullet? It seems like the lighter bullet would be flatter shooting even though it does have .015 less BC. What gives?

Does the velocity for the 130gr bullet seem reasonable? I was hoping for around 3150 fps, but I am not stressing about 100 fps.

Any input would be appreciated. Thanks!!!

mehavey · December 16, 2010, 10:00 PM

Raising the "sight height" in the ballistic calculator effectively raises the line-of-sight to the target. This means you have to (effectively) tip the rifle "up" to have the bullet intersect that higher line-of-sight. The bullet is then traveling a steeper parabola and continues to rise for a longer period of time/distance before starting back down. It actually goes through "zero" twice -- once on the way up (EX: 140yds) and once on the way down (EX: 251yds).

In the case of the lighter bullet, that lower-BC 110 grainer may initially go out faster, but it also slows down faster. Hence the lower yardage MBPR

December 16, 2010, 09:01 PM	#1
Tex S Senior Member Join Date: November 12, 2008 Location: Fort Worth, TEXAS Posts: 909	Taller sights will increase MPBR? I have been using my new chrony and it seems to be working well. I finally got to test some loads I made for my 270 win. Here is the load... Barnes 130gr Tipped TSX with a BC of .392 60gr of H4831 (careful, this is a max load in my Lyman manual) Fed 210M primers Average velocity is right at 3050 fps So I was wanting to calculate the MPBR of this load. I went to the JBM website and started playing with their ballistic calculator. I think I have everyting entered correctly, and it looks like the MPBR for this particular load (with the sight being 1.5" off the bore and a 6" kill zone) is 296 yards, with a zero of 251 yards. I have found that if I increase the sight height the MPBR increases a fair amount. This baffles me. What am I missing? I also noticed that if I switch to the Barnes 110gr Tipped TSX bullet (BC .377) and keep every other field (including velocity) the same, I actually lose yardage on my MPBR. Is this due to the lower BC of the 110gr bullet? It seems like the lighter bullet would be flatter shooting even though it does have .015 less BC. What gives? Does the velocity for the 130gr bullet seem reasonable? I was hoping for around 3150 fps, but I am not stressing about 100 fps. Any input would be appreciated. Thanks!!!

December 16, 2010, 10:00 PM	#2
mehavey Senior Member Join Date: June 17, 2010 Location: Virginia Posts: 6,894	Raising the "sight height" in the ballistic calculator effectively raises the line-of-sight to the target. This means you have to (effectively) tip the rifle "up" to have the bullet intersect that higher line-of-sight. The bullet is then traveling a steeper parabola and continues to rise for a longer period of time/distance before starting back down. It actually goes through "zero" twice -- once on the way up (EX: 140yds) and once on the way down (EX: 251yds). In the case of the lighter bullet, that lower-BC 110 grainer may initially go out faster, but it also slows down faster. Hence the lower yardage MBPR