Results from 7mm and 222

[remingtonman_25_06]

Been having some complications with my 7 RM and 140g NBT lately. Seems either the BC is lower then advertised, my scopes clicks are not true .250", or the velocity really isn't 3250fps as per my Chrony. Heres the problem. I was suppose to be 59-60 clicks up from a 200y zero to be dead on at 800 yards. Well turns out I need about 73-74 clicks. I checked the velocity and zeroed about 10 minutes before I shot 800, so I know it was zeroed and I know what the crony said for velocity. Things just didn't/dont match up. I think I foudn the problem. I worked backwards/forwards and every which way. First I was told to take a 100y zero and dial 20 MOA or 80 clicks up and see where they land at 100 yards. If my scope was true 1/4", my shots should land 20" high. Well they landed 20.5" high which is close enough. Test #1 success. I was then told to shoot at 400 yards from a 100y zero and see where my bullets actually drop on paper and measure the distance. According to my program, 140g BT at 3250fps with BC of .485, would put me 20" low at 400 yards. I shot a nice 1" group at 400 yards that was 23" low. SO obviously right there is my problem. Velocity. That cheap a** crony I have has got to go. If I'm 3 clicks off at 400, its only going to get worse and thats why I was about 11-12 off at 800. If I enter 3075fps and .485 BC, this matches me up pretty much to the T at 400 and 800 yards. According to my drops and clicks anyways. My advice is for anyone that has a cheap Crony, beware of what they tell you, its probly not true. I tested a couple 222 rem loads w/55g NBT with I4198 and I4895.

Heres the 400y group fired with my 7 RM and 140g NBT over 68.5g RL-22, W-W case, 3.350", crony says 3250fps, actual drop and shooting says 3075fps is more like it :x One of my better 400y groups though!!

23g I4895 shot pretty good, .6" for 5 shots right at 3000fps. Will see how she does on a few yotes!!! 18.5g I-4198 shot decent at .8", also right at 3000fps. Seems the 222 likes the 55g at 3000fps. Had her up to 3175fps, but accuracy was not as good.

 

[Guest]

I suspect the Chrony is not the problem. I imagine that the BC figures are calculated, and thus, not absolutely accurate. I say this because Nosler has recently begun to publish actual, tested BC's for some of their bullets (see the 8mm bullets) versus the calculated (software simulation generated) for some of their others. I can tell this, because the BC for a 200gr .323 AccuBond is lower than that of both the .308 (which it should be) and that of the .338 (which it should exceed, based on similar weight and larger diameter, thus a shorter overall bullet). I suspect if you set up your chrony and another chronograph either overlapping (like I can do with my chrony and PACT units) or back to back, the velocities would be similar enough to attest to my analysis. Also, if you are comfortable setting up the chrony downrange, use it at 100yds, and see what the average velocity is there, for this known load. I suspect it is below what the ballistic calculator is telling you, indicating a BC problem, not a chrony problem.

 

[remingtonman_25_06]

I dont think BC is the problem dubyam. I have discussed this over on LRH and couple guys use the bullet and they have said that out to 700 yards, the .485 BC has matched up quite well. Noslers are usually pretty close to BC. Now my BC would have to be around .350 for me to be shooting that low or having to put on that many extra clicks. I really highly doubt that its that low. I still think its the crony. I zeroed at 100 and shot at 400 yards to confirm drop. If velocity was 3250 it wouldn't ahve dropped 3" more then it should. Shooting was done off a bench so rock solid rest was used. All testing I have done so far leads me to believe my crony is giving me high readings. I am experiencing this problem at 800 yards with several other caliber/ combos. They always shoot anywhere from 2-4 MOA lower then they should at 800 yards. A lot of others have had this problem with the cheap crony's so go figure. I will be looking into the Pact cronographs in the near future.

 

[Guest]

I get pretty much similar velocities from my Beta Master Chrony and my PACT Professional, so I don't understand the issue. Perhaps I am just lucky. I will retest here in the near future just to verify my readings are compatible, but so far, so good. If you get a PACT, you can zip tie the chrony onto the mounting bar (centered between the PACT screens) and get a really good idea of how far off the velocity is, if at all. For the record, Nosler used to publish a BC of .401 for the 8mm 180gr BT, and now publish a corrected (from actual testing) BC of .357 for the same bullet, so significant variations can occur, although not quite of the magnitude you indicated in your post. It is certainly a problem to be investigated. Do you know anyone with a PACT who would let you test first, before you have to spend the money? Might be worth the trouble. If you were closer to Alabama, we might could shoot together sometime, but you are not close, I think.

 

[baltz526]

elevation and humidity of your test location is the most likely issue you are having.

 

[Guest]

Would elevation hurt BC, or just affect velocity? How about humidity? Seems like the bullet will have a lower BC in high humidity situations, just because there are more molecules in the air to create drag, but I am no physics whiz.

Of course, if the published BC is calculated, not test derived, and then altitude, temperature, and humidity also affect BC, maybe one could see a drop from .485 to about .350. I am still curious to hear about the comparitive velocity test, if Rem is able to do it.