Was all good but today shooting low?

[Bigcat_hunter]

Ok so just fine tuning this new gun for fall elk I have a question. So chrono speed averages 2750 fps but with the dope the last few shooting sessions I had to adjust the speed up to 2800 fps to shoot where it is supposed to from 500 to 1100 yards so I assumed its all good.

Today it is shooting low, more like it is actually 2750 fps. Should I have adjusted the BC instead of the velocity? What would cause this to suddenly shoot low? Scope is zeroed I checked. The temp was 20º warmer today then the last few sessions but I entered that into my dope.

What say you??

 

[FearNoWind]

"... I had to adjust the speed up to 2800 fps to shoot where it is supposed to from 500 to 1100 yards ...

So then it's apparently not shooting where it's supposed to -

How "low"' is low - compared to previous groups?
What's the ES?
You may have simply moved it out of its node while looking for a higher MV.

 

[Bigcat_hunter]

So then it's apparently not shooting where it's supposed to -

How "low"' is low - compared to previous groups?
What's the ES?
You may have simply moved it out of its node while looking for a higher MV.

No I didn't adjust the load at all. I adjusted the info going into the ballistics program to get the dope to the right place. Shoots about 1 MOA low.

 

[MMERSS]

What bullets are you shooting?

 

[Bigcat_hunter]

Berger 210 vld 300 rum.

 

[Engineering101]

bigcat_hunter

If you know the velocity is correct you change the BC not the velocity. If you change the velocity your time to impact will be wrong as will the kinetic energy you are dumping into your target as will the terminal velocity. All more important to know that some three digit number that you don't otherwise use.

Do you know your chrono is giving good data? How do you align it? Any misalignment in either pitch or yaw will cause it to read low. To quantify that, 10 degrees misalignment in both pitch and yaw will cause it to read a bullet going 3,000 fps as 2,900 fps.

Also, why so slow? I'm running the same bullet 300 fps faster.

 

[Bigcat_hunter]

bigcat_hunter

If you know the velocity is correct you change the BC not the velocity. If you change the velocity your time to impact will be wrong as will the kinetic energy you are dumping into your target as will the terminal velocity. All more important to know that some three digit number that you don't otherwise use.

Do you know your chrono is giving good data? How do you align it? Any misalignment in either pitch or yaw will cause it to read low. To quantify that, 10 degrees misalignment in both pitch and yaw will cause it to read a bullet going 3,000 fps as 2,900 fps.

Also, why so slow? I'm running the same bullet 300 fps faster.

Powder shortage is why I am shooting slow. H50 bmg, 95 grains is what I have. I have adjusted velocity in the past and it worked so thats why I am aking. I have never adjusted BC for a correct drop.

 

[Engineering101]

bigcat

I feel your pain. I'm running 97 grains RL-33 which got me 3,066 fps the last time I pulled the trigger but there isn't a whole lot left in the jug. If things don't get better soon, I'll be joining you in the 2,750 fps club as they do have 50 bmg at the local shop.

Another reason to adjust BC. Your rifle is mangling the bullet in a way unique to your load/rifle/powder which will effect the BC of the bullet. The bullet box may give you a number for BC but that may not be what you get when it comes out the end of the barrel. Also Berger is one of the manufacturers that uses ICAO standards to establish their BCs as opposed to Std Metro conditions. Those with ballistic programs using std metro (like me) need to multiply the BC by 1.018 to correct for that difference. The same is true for Nosler and Lapua.

 

[Broz]

Still got the EGW rail on it?

 

[bigngreen]

I'm very reluctant to change a solid BC unless I can verify over several ranges with consistent results. Being on one day then days later being a little high or low I really look for an environmental reason. A switch of wind direction is a main one, baro or temp also.

 

[Pdvdh]

Today it is shooting low, more like it is actually 2750 fps. Should I have adjusted the BC instead of the velocity? What would cause this to suddenly shoot low? Scope is zeroed I checked. The temp was 20º warmer today then the last few sessions but I entered that into my dope.

What say you??

If your ballistics program matched one day, it should match again on another day under similar environmental conditions/air density, even if your BC or MV is incorrect. If your BC and/or MV is errant, then a substantial change in air density could yield differing results with the different air density. There's really no way anyone could know what's up, based on the limited information available to us. There many things that could be in play, including operator error with the ballistics program or scope turret. It sounds like you completed your first long range drop session with that load, and the second time out, you had different drops. I'd shoot it a few more times, in windless conditions.

I'd tend to trust Berger's BC value and tweak the MV, unless you've got a rock solid chronograph.

 

[MMERSS]

Berger 210 vld 300 rum.

Other members have already replied regarding looking elsewhere before adjusting Berger's BC's. I would concur, look toward adjusting velocity first.

 

[Bigcat_hunter]

It was quite windy and humid compared to the other time. I shot. Maybe that had something to do with it. I have never used the barometric pressure in my dope, just the altitude.

 

[The Oregonian]

If you raised the velocity in the program but not in actuality and it is now shooting low, wouldn't that imply that it would be on at a lower velocity? You said it was shooting more like 2750 fps....

Or am I missing something?