Wildly optimistic BC's?

[Linefinder]

I'm a little stumped here, and I'd appreciate a little advice.

I took one of my .223 Rems out to the range today for a little load development/testing. I was using 55 grain VMaxes pushed by 4 different charge weights of H322. My shooting routine was as follows...

,5) rounds across the chrono to get average velocity.
,5) rounds for group at 100 yards.
,5) rounds for group at 200 yards.

I did this for all four recipes. The lowest charge yielded an average of 3102 fps, with the highest charge clocking an average of 3426. The chrono was located 15 feet from the muzzle. I was shooting at approximately 6300 feet ASL, in 48F ambient temp.

I was not so much interested in where the group landed on the target, as much as how much lower the 200 yard group impacted below the 100 yard group while using the same aimpoint and making no sight adjustment.

In general, the 200 yard groups impacted 3.2" to 3.7" lower than the 100 yard groups.

The Hornady Manual lists the BC of the .224 55 grain VMax as .255. In order to get my Oehler Ballistic Explorer software to match the trajectory I actually obtained, I had to adjust the BC value down to .12....IOW, a 50% reduction in BC. I've heard of "tweaking" BC numbers, but this seems excessive.

I have gone back through all my data entry, and for the life of me, I can't see any input errors.

Is this kind of BC "variance" common, or am I overlooking something obvious?

Thanks

Mike

 

[memilanuk]

Couple of thoughts...

1) Chrony's,as in Shooting Chrony) have been known to be a bit... 'optimistic' in terms of velocity readings. It's been a good while since I ran light bullet .223 Rem loads across a chrono; 3400+ seems a tad high. The drop is just about right for 3000fps, though. That's an awful lot more error than I would expect, though.

2) Sight height can fudge things a bid as well.

3) Actual barometric pressure can deviate significantly from the pressure derived strictly from the altitude,which is all the altitude is for). However, that shouldn't really be noticeable for several hundred more yards.

Sorry I don't have a good answer for you, hope you get it figured out.

Monte

 

[Linefinder]

Thanks for the insight. I will admit, I am using a new Shooting Chrony, but it has clocked my "old" loads at pretty much the same velocity as my PACT unit. I just bought the Chrony because it's not always convenient to set up the PACT unit.

And yes, the 3400+ load is a little fast,,and a little over published "max") but I carefully worked my way up to it with the "assistance" of QuickLoad and keeping a close eye out for pressure signs along the way.

Even if the MV's were 150 fps wrong,which I don't think they are), it still shouldn't cause that much difference in the required BC correction, I wouldn't think.

Mike

 

[Cheechako]

Mike

There is obviously something wrong here, as you already know.

If it was me I would change one of the two things in question. The bullet or the chrony. It will involve some extra work on your part but until you eliminate one of the potential culprits you'll be chasing your tail until spring.

Good Luck

Ray

 

[Cheechako]

It's very true that scope height is critical but I think we have to give Mike the benefit of the doubt and assume that he has made all of those critical measurements and that his input is correct. If so, then there is something else at play here. Personally I don't think it's the bullet BC but rather the chronograph. But there are ways, as I have suggested, to determine that.

It's amazing how different ballistic programs yield different results. Running Gunamonth's figures thru the JBM program yeilds drop figures nearly 1/2" different.

Ray

 

[Linefinder]

Scope height is keyed in correctly. Two things come to mind.

I didn't actually "zero" for 100 yards, and the 100 yard groups were impacting from .75-1.0" below POA at 100 yards. I didn't figure that mattered, since all I was trying to ascertain was the difference in drop between 100 and 200 yards. That may very well have been my first mistake....or would it, since I was using the exact same aimpoint?

The other thing is probably a real stretch, but....when doing load testing or pdog shooting, I always wear prescription sunglasses. Yesterday, I forgot them at the house and wound up conducting my test while wearing a brand new pair of "progressive tri-focals". These glasses have a really weird hour-glass shaped "sweet spot". I have to hold my head in different positions to focus on things of different sizes and at different distances. I've never worn them when shooting before.

If my parallex adjustment was a smidge off, and I was having to "juggle" to achieve a decent sight picture....well, you see where I'm going with this. Like I said, this is probably a stretch, but it could have been a contributing factor.

It's beginning to seem like I have to re-do the test.

Mike

 

[Cheechako]

Mike

Before going out and re-doing your tests, do some reverse-engineering with your data. Work with it until you get the POI at 1" low at 100 yards and see what you come up with. Your 200 POI is going to be at least 1" lower than what you predicted and you may find that it's probably pretty close to what you actually experienced and that neither velocity nor bullet BC is too far off.

BTW, I wear tri-focals and as long as you eliminate parallax they should not be a problem.

Ray

 

[Linefinder]

Why in the heck I didn't think to do that, taking into account the 1" low at 100 yards?:thumb: That is going to mean that in actuallity, I probably had a 50 yard, or so, zero....then the 200 yard drop is going to be in line.

Man, I tell ya.......sometimes I can't see the forest for the tree.

I've gotta get to work, so I'll run the numbers tonight and post them back here.

Thanks for the help, guys! I really appreciate it.

Mike

 

[Linefinder]

Thanks, gunamonth.

You're right. There's no way physically possible that my bullet ever crossed the LOS. Considering I've thumped almost a thousand pdogs with this rifle, all I can figure is that while "re-indexing" the elevation turrent after having the action trued, I must have missed it by a mile.

Boy, I feel like an idiot. But, on the upside, soon I'll have made every mistake there is to make, and maybe I can save you guys some time one day!

Thanks for all the responses. I certainly appreciate them.

Mike

 

[Linefinder]

If you were an inch low at 100 yards you were probably not zeroed at any range. That bullet at that speed only drops about 1.75" from the bore line at 100 yards so to be an inch low it could never have crossed the line of sight of the scope.

Just wanted to say thanks again. You nailed the problem exactly. I went back out today for further testing and did a "proper" 100 yard zero. Drop at 200 yards was 1.6". If anything, the BC seems to be a little better than the published data. OTOH, it could have been parallex....LOL!

I appreciate the input!

Mike