I figure this may be useful to a few people. I have been working on a
project to match my TA01NSN Bullet Drop Compensator trajectory (based
off the M855 load) with a 75gr bullet out of my Bushmaster M4A3 (14.5"
1:9 chrome lined barrel).
This post provided a bunch of data to get me started: http://www.ar15.com/forums/topic.html?b=3&f=18&t=229420&sr=0
For this project I chose the Hornady 75gr BTHP-Match .224 bullet and Hodgdon Varget powder.
Using the trajectory calculator at http://www.eskimo.com/~jbm/cgi-bin/jbmucard-5.0.cgi
I came up with the following potential trajectories (MOA). (The M855
trajectory I based on a 62.0gr bullet with BC .304 moving at 2970fps.)
Potential trajectories (MOA)


Potential trajectories (inches)

The "Hornady 2600 fps +2" ZERO" trajectory seemed to be the best
trajectory and most feasable. The challenge is therefore to get 2600
fps with Varget out of a 14.5" barrel.
Here is my chrony results for 24.0gr - 25.5gr at 0.5 increments
(WARNING: THESE ARE HOT, COMPRESSED LOADS! START LOW AND WORK YOUR WAY
UP!). All loads were lightly crimped (Lee Factory).
Little Boy (Bushmaster M4A3 14.5" 1:9 Chrome Lined)
2009-04-25   50 degrees
Hornady 75gr BTHP-M 10-shots
Varget 24.0
Winchester Brass CCI-400 Crimped OAL=2.25
Lo 2471
Hi 2514
Av 2514
ES 71.11
SD 23.87
Little Boy (Bushmaster M4A3 14.5" 1:9 Chrome Lined)
2009-04-25   50 degrees
Hornady 75gr BTHP-M 10-shots
Varget 24.5
Winchester Brass CCI-400 Crimped OAL=2.25
Lo 2572
Hi 2599
Av 2583
ES 27.06
SD 09.74
Little Boy (Bushmaster M4A3 14.5" 1:9 Chrome Lined)
2009-04-25   50 degrees
Hornady 75gr BTHP-M 10-shots
Varget 25.0
Winchester Brass CCI-400 Crimped OAL=2.26
Lo 2571
Hi 2647
Av 2621
ES 75.90
SD 22.24
Little Boy (Bushmaster M4A3 14.5" 1:9 Chrome Lined)
2009-04-25   50 degrees
Hornady 75gr BTHP-M 10-shots
Varget 25.5
Winchester Brass CCI-400 Crimped OAL=2.26
Lo 2682
Hi 2710
Av 2699
ES 28.23
SD 10.34
**Signs of pressure!
The 25.0 gr of Varget
produced and average velocity of 2621 fps, my goal velocity. Surprisingly, 25.5 gr of
Varget produced 2699 fps, but... this
load showed signs of pressure with flattened primers; it was also
difficult to get 2.26 OAL because the load is so compressed. Therefore,
I am excluding this load as a viable option.
Based off this data I am choosing to go with the 25.0gr Varget load in my M4.
Trajectory of 25.0gr Varget load vs M855 trajectory (inches).

I am not the greatest shot but accuracy appears to be decent. Here is
the target for the 25.0gr Varget load (10 shots taken from a bench,
bi-pod in front, sand bag in back, 100 yards, TA01NSN optic). Using
another 10 shots of this load I was able to ring the 400 meter gong on
all 10 shots.

Summary: The trajectory of a Hornday 75gr BTHP-Match bullet through a
Bushmaster M4A3 (14.5" 1:9 chrome lined barrel) best matches the
Trijicon ACOG TA01NSN Bullet Drop Compensator when sighted in 2 inches
high at 100 meters and backed with 25.0 grains of Varget. This load
produces 2621 fps giving a trajectory difference between the
TA01NSN BDC trajectory of:

Range (Meters)	Difference (MOA)	Difference (inches)
100	1.7	2
200	1.1	2.7
300	0.6	2
400	0.1	0.5
500	-0.3	-1.5
600	-0.4	-2.9

Raw Data: http://media.ar15.com/media/viewFile.html?i=9742






 

nice post thanks

Good stuff.....what makes this such a great site.  Working up the same for my 20" 1:8 Rasmussen Arms.  Thanks!