Velocity Spreads

[Hicks]

Ok, another newbie question. I was shooting this weekend and this outing was really the first time that have ever taken any data. 7mm RUM factory CDL, 26" barrel, yada yada. I'm loading, on advice I received here, 92.5 gr of Retumbo. No signs of too much pressure, and the temp was in the 70's, but over 10 shots I got a difference of more than 120 fps (3230 to 3350 ish, I don't have the data in front of me at the moment). I'm guessing this is not good? I then switched to a '06 with a 26" barrel and got a spread of more than 60 fps with I think RL 22. My question is, all things being equal, what is a good velocity spread?Hicks

 

[steve smith]

A good velocity spread would be right around 0-fps. A more realistic spread would be around 30fps or less. Or at least thats been my experience.

Although! In general, the larger the case capacity the more likely you are to have extreme spreads in velocity. Smaller capacity cases make it easier to find that load combination that produces the best results. They seem to be more forgiving in that respect. Again, that has been my experience.

 

[Boss Hoss]

If you let your rounds sit in a hot chamber for more than 3 seconds or so then heat is also playing a possible part in this. You cannot let the rounds begin to cook in the hot chamber. If you have trouble getting the sight picture then take the round out and put it to the side and shoot it later and insert a fresh one.

I shoot 1K and my high to low must not be in excess of 25fps preferably less than 10fps.

 

[britz]

I used a Prochrono and currently use another $80 chronograph. I shoot a 300WSM and I have been geting bad velocity spreads as well. I atribute most of it to a cheap chrono and some to weather conditions. I also use mine mostly for establishing a average velocity to use in terms of a long range drop chart. I NEVER use it to determine how good my rounds are.

 

[Eaglet]

I used a Prochrono and currently use another $80 chronograph. I shoot a 300WSM and I have been geting bad velocity spreads as well. I atribute most of it to a cheap chrono and some to weather conditions. I also use mine mostly for establishing a average velocity to use in terms of a long range drop chart. I NEVER use it to determine how good my rounds are.

A good chono is a powerful tool in the hands of a reloader. And it should give you an indication of how well tuned and how good your ammo is. Specially if you're doing some long range shooting. Two bullets with 50 ft/sec difference in a flat shooting rifle, would give you a vertical distance of about 8+ inches using a bullet with very high BC; to that we add the wind and human error etc. That'd be bad business. Now, if you're limiting your shooting to about 300 yards and the rifle will group then you don't have to worry about it. JMO.

 

[britz]

I do agree with you Eaglet, however, I don't think that a Pro chrono or any $80 chrono will be accurate enought to rely on for telling you how good your loads are. You should rely on the accuracy of your groups if you don't have a exceptional chrono. The Pro Crono admits that it is only accurate to 15 fps to start with.

 

[AJ Peacock]

If you let your rounds sit in a hot chamber for more than 3 seconds or so then heat is also playing a possible part in this. You cannot let the rounds begin to cook in the hot chamber. If you have trouble getting the sight picture then take the round out and put it to the side and shoot it later and insert a fresh one.
...

+1

I was shooting last weekend and was seeing VERY low deviations (extreme velocity spread of less than 10fps for entire strings) on my 7mmRM. For an experiment, I loaded a round in the hot chamber, waited 30+ seconds and whammo 30-45 fps difference (did it a couple times). Let everything cool down shot faster and back to miniscule velocity spreads. I'm more interested in first shot velocity, so I like to shoot, wait 3-5 minutes then shoot again.

Hope this helps,

AJ

 

[davewilson]

a 7rum is considered an extremely overbore cartridge. generally speaking, the more overbore you get, the harder it is to keep the ES down.

Retumbo is great for velocity, but not sure i've ever heard anyone getting their lowest ES with it. usually it gives higher velocity spreads.

 

[stxhunter]

i worked up a load of 95.5 gr of retumbo and 200 gr ab 3135fts for my 300 rum that has es of 29fts avg of ten shoots. but i might of let a couple of them cook a little in the chamber i'll take that bit of advice into consideration next time i chrono my loads thanks

 

[Eaglet]

a 7rum is considered an extremely overbore cartridge. generally speaking, the more overbore you get, the harder it is to keep the ES down.

Retumbo is great for velocity, but not sure i've ever heard anyone getting their lowest ES with it. usually it gives higher velocity spreads.

Dave,
I have two 300 RUM, all I have used is Retumbo, lots of Retumbo. Keeping the ES 15 or less has not been an issue and many times it will go in the single digits. It works good wood Federal and CCI primers. As you know, to get single numbers you have to always keep in mind not to cook the ammo in the chamber after the firs shot, specially when shooting hot loads.

 

[davewilson]

Eaglet, know of 2 others that get good results with Retumbo in their 300rum's. i've tried it in a couple big cases and know a few others that have also and like i said. generally speaking, it gives a lot of velocity but higher spreads. it's great to have several options. makes reloading much more enjoyable.

speaking of options.....Eric, where are the 338 bullets?