confused and need help

[skuna boy]

I have a 300 RUM with 26" barrel and since the powder shortage have ran out of retumbo so have been looking for something else to make due until I can find some. Since 50 BMG is so easy to get I figured why not and worked up a load doing a simple ladder 20 shot ladder test, starting in the low 90's and worked my ending at 102 grains. best grounds were around 99 and 101 grains so I went with 101.5 grain load and get sub .5" groups at 100 yards with this load.

The problem comes in when I start doping and shooting at longer targets. Looking at other's load data in the internet and reloader manuals this load should be around 3000 ft/sec. I plugged this into my iSNIPE app and G7 software and both produce about the same dope. When I shot using the 4.7MOA adj at my 325 yard plate, I hit approximately 48-60" low!!!!!!!!!!! which is hard to pin point exactly due to the lay of the land and the fact that it covered in grass and weeds....but I have seen dirt fly up short and found where I am hitting. So I started decreasing the app FPS setting by 100 and dialing that and have gotten down around 2000 FPS and 10 MOA dial up....

I have triple checked to make sure my scope it tracking and it appeared to be. it's a mark 4 and fairly new. I triple checked my powder scale to make sure my charge was not lighter than I thought it was.

Could it be that this 50 BMG is performing that poorly? could I have a bad batch.

I have some 7828 and H1000......not much but some....I'm going to load some 7828 and see what happens but would like to know thoughts from folks that know more about this stuff than I do.

 

[MMERSS]

Sub .5 MOA groups at 100 with a RUM would be exactly where I would want to be. Your powder is not the problem. Shooting 40" low just over 300 yards after sub .5 MOA groups at 100 indicates there is another problem. Double check all your mechanics (base screws, scope rings, scope tracking, etc.)

Variance in powder will not cause poor precision from less than 1/2 MOA to over 10 MOA in just over 200 yards. It's something else.

 

[Engineering101]

Most likely it is running much slower than 3,000 fps with your lot of powder in your rifle. Chrono the load.

 

[skuna boy]

This was my first thoughts too...I double checked everything on the mounts and all good, unless something internal in the scope....but if it were mechanical.....would it not effect shots at both 100 and 300? Even at 300 it is consistent....just way low.

I ordered a chronograph yesterday.... don't have access to one and tired of guesses speeds so we will see when it gets here.

 

[MudRunner2005]

Sub .5 MOA groups at 100 with a RUM would be exactly where I would want to be. Your powder is not the problem. Shooting 40" low just over 300 yards after sub .5 MOA groups at 100 indicates there is another problem. Double check all your mechanics (base screws, scope rings, scope tracking, etc.)

Variance in powder will not cause poor precision from less than 1/2 MOA to over 10 MOA in just over 200 yards. It's something else.

That was the first thing I thought when reading this....Scope or mounting issue.

 

[MMERSS]

.....would it not effect shots at both 100 and 300? Even at 300 it is consistent....just way low.

Try 200 and see what you get. I hate to state the obvious but did you turn elevation in the right direction? Just asking because I've been there before. Some scopes elevate clockwise rotation opposite to what we may be accustomed to.

 

[FearNoWind]

Most likely it is running much slower than 3,000 fps with your lot of powder in your rifle. Chrono the load.

Without doing that you're chasing smoke in the wind.

 

[Kennibear]

You never mention the bullet weight/make.

Most people at this level own a chronograph. It takes the guess work down a couple of notches. The ES and SD data is also very helpful.

Retumbo is quite a bit faster than H50BMG. Given that the 300 RUM uses a case full (or thereabouts) of Retumbo the velocity with H50BMG will not match Retumbo without compression, and probably a lot of compressed H50BMG. Hodgdon's lists 102gr of H50BMg and 95.5gr of Retumbo for the 308 Nosler 200gr AB. The Retumbo is @ 62.5Kpsi and the H50BMG is @ 58.9Kpsi. The H50BMG is 100fps slower. Both are compressed.

In another Hodgdon's table with the Hornady 190gr SpBT max for both is:
105gr H50BMG (compressed), 3085fps, 63Kpsi
96gr Retumbo (NOT compressed), 3204fps, 62.5Kpsi

Note that H50BMG was higher pressure and 121fps slower.

Use the chronograph to solve your questions.

KB

 

[coyotekiller82]

Try 200 and see what you get. I hate to state the obvious but did you turn elevation in the right direction? Just asking because I've been there before. Some scopes elevate clockwise rotation opposite to what we may be accustomed to.

^^This...turn the dial the other way!!

Probably something all of us have done at one time in our shooting life