Parker BE Bullet Ballistic Coefficient

[winelines]

I have a Remington Ultimate ML converted by Jeff Hankins to a .45 SML. I am shooting sized Parker 275 gr BE bullets - no sabots, no lube.
I am using 68 gr of 4198. My chrony readings average around 2790 with small (10-15 fps for 3 shots) variations. I do see some variation (1.85 fps/deg) with prevailing temperature. I am getting very good precision with this load (less than 1 MOA out to 500 yds on windless days), however my actual trajectory varies greatly from a calculated trajectory using a G1 drag model, the above velocity, and the published BC of .296 (which is quoted for 2850 fps). I don't think I have a chrony problem because trajectory results for all my other rifles are very close. If I use the measured velocity and vary the BC, a BC of .21 results in a trajectory calculation that comes close to matching my observed shooting results. That variation in BC seems unusually large. My experience with many other bullets in a number of guns has shown a much closer correlation with published BCs.

So, my question is - Has anyone using Parker BE bullets seen a similar result? Is this just the result of the BC changing significantly because of the large drop in velocity over 500 yds (down to 1130fps) or is something else going on? I know the BC varies with velocity, but if it starts out at around 3, it has to drop a long way to average .21.

[rangeball]

winelines, welcome

I think it's been noted that the published parker BCs are generous, as your real world experience has proven.

I would suggest you move this post to the main smokeless board as it gets a bunch more traffic.

[winelines]

Will do, but since I'm new I haven't a clue on how to do it. Some help would be appreciated.
Also what is the official title of the "main smokeless board"? I don't see one listed by that name.

[edge]

Moved by magic

Can you post what parameters you used to calculate the BC?

edge.

[edge]

If you think you are down to 1130 fps then you might want to shorten that distance a bit as you are in the transonic range.

FYI if I plug in 2790 at the muzzle and 1130 at 500 I get a G1 BC of 0.259 at standard sea level conditions ( 59 F, 29.53 HG, and 78% Humidity )

edge.

PS are you dialing the scope or measuring drop using your 100yd range?

[winelines]

Thanks for the move.
I didn't calculate the BC, I just kept reducing the BC and running the ballistic program (Shooter) with the actual alt/temp/hum (it was a hot humid day at sea level) until I got a trajectory that matched my real world results.
I am aware of the low velocity and low energy. I had a chance to shoot at that distance on a windless day, so I to took it as an experiment. Got good precision but I wouldn't shoot at anything at that distance.
I just found a post on modernmuzzleloader and found a report that some actual velocity testing had demonstrated a BC for the that bullet at .21!!! I should have researched more before I asked the question.

Thanks for your help.

[edge]

My only problem with the 500 yds is for BC calculation not for shooting
If the bullet is transonic the BC may change drastically from supersonic to subsonic so you might not get an accurate BC.

edge.

[GMB54-120]

Sounds like you have the chrono and the range to shoot them. Why not shoot them over a chrono at various ranges and get your real world BC.

Ive seen as low as .214 BC when shot from a Savage with 40gr of SR4759. Granted he only measured velocity at the muzzle and 100 yards.

[winelines]

Understand. That day I shot 3 shots at 100, 200, 300 and 500 yards in that order.

And yes, I was dialing the scope and holding center based on a calculated trajectory. I then measured the distance between the calculated and actual group centers. On a subsequent day (different altitude/weather) I shot at 100, 250 and 350 and got similar results.

[winelines]

Interesting thought GMB; but I think setup would be pretty difficult outside of 200 yds. Its a public range - generally busy. I think I got what I wanted. Namely, a trajectory I can match with actual results out to 400 yds. I just use a BC of .21

[bestill]

I have no technical explanation but i use.250 for parker275 BE to get accurate drops from100 to 500

[winelines]

What is the initial velocity?

[bestill]

My load totally different2425fps.

I get my true drops at range then change bc of bullet to match drops my reason is i use ballistic chart mostly for temperature changes and wind adjustments.

[edge]

IMO since you have the range, shoot at every range you can without changing the scope.
Let's say you have 36 inches of drop, then you need a target with the spot and at least 36 inches below to record the hit.

This confirms both the BC and your scopes ability to track in clicks and eliminates and click error

edge.

[winelines]

Thanks Edge, I'll do it.

[jsthntn247]

I had to put .240 G1 to get mine to match out to 500 yards.