Importance of shot string for chrono results

[leftyk82]

I'm wondering the following:

- What is the general consensus on ideal number of recorded shots to get MV averages and SD's?
- Are shot strings to include the cold bore shot?
- Should there be a specific time lag between shots to keep barrel temperatures similar?

My Magnetospeed numbers for the Hornady 140g Factory 6.5 Creedmoor are below, eight shot string was not quite rapid fire but probably only 15 seconds between shots:

Shot 1 2737 (cold bore)
Shot 2 2766
Shot 3 2755
Shot 4 2785
Shot 5 2761
Shot 6 2805
Shot 7 2761
Shot 8 2787

Yields avg of 2769 and SD of 21.4

24" barrel
75 degrees
29.27 station pressure
RH 77%
DA 2055

Thanks in advance.

 

[_Windrider_]

That’s a lot better than I get at 75 degrees with same ammo

 

[BigTex]

Most people on the Hide use 3 shot strings to report SDs from what I can tell, I prefer 1 shot strings myself.

Honestly 10-20 shots should give you a good statistical sample.

 

[Deleted member 10043]

Depends on the rifle and chronograph Generally, 10 shots within five minutes. Keep track of the strings on a spread sheet until you get to a 90% confidence level. If you don't like statistical sampling then a total of 30 will get you in that ballpark. But ammo is expensive so something like investing in a magnetospeed is worth the investment and will require less data for our purposes.

 

[leftyk82]

Forgot to add that I shot with a can on; would that matter?

 

[BigTex]

Nope

 

[leftyk82]

That’s a lot better than I get at 75 degrees with same ammo

Picked up a different box from a different retailer and that eight shot string averaged 2714. Pretty big variance from my initial results. Similar weather conditions.

 

[_Windrider_]

Picked up a different box from a different retailer and that eight shot string averaged 2714. Pretty big variance from my initial results. Similar weather conditions.

i get 2701 with the same ammo.

 

[Rerun7]

I usually do 10-15 shots to get initial numbers for the ballistic calculator. From there I true the dope by shooting at distance.

 

[Deleted member 113831]

The shot cadence depends on what you wish to accomplish. Barrels often trend one way or the other as the barrel heat increases. I have three rifles that consistently lose velocity as the barrels heats up. This would be important to a match shooter who wants to know what his barrel will do when 12 rounds go down the tube in 90 seconds.

 

[kabarNC]

-Should there be a specific time lag between shots to keep barrel temperatures similar?

I've been wondering the same thing, but in regards to cartridge temperature. I wonder if cold bore shots are different because chamber temperature is ambient, and subsequent rounds sit in a hot chamber and cook for a bit. Could it make a difference, even in powders that aren't very temp sensitive, if after cold bore was out of the way, to be consistent with dwell time of live rounds chambered? I feel like it could, but would like to hear more..

 

[Deleted member 113831]

I've been wondering the same thing, but in regards to cartridge temperature. I wonder if cold bore shots are different because chamber temperature is ambient, and subsequent rounds sit in a hot chamber and cook for a bit. Could it make a difference, even in powders that aren't very temp sensitive, if after cold bore was out of the way, to be consistent with dwell time of live rounds chambered? I feel like it could, but would like to hear more..

In regards to cold bore shots vs hot barrel shots, there are a few diferent things in play. Barrel lubricity is diferent for a cold bore shot because either the barrel is clean, or because the powder residue in the barrel has had time to absorb moisture. Whatever copper fouling might be present may have had time to oxidize, also affecting lubricity. I have found that If I run a bore snake or a dry brush followed by a dry patch, that my cold bore shots will most often, fall within the group.

Shots from a hot barrel will be affected by chamber heat if allowed to cook. I haven't been able to determine if this is due to the powder heating up, or if it is the heat affecting the primer mixture. The other thing is barrel expansion and contraction with heat.

The barrels I have seen that lose velocity toward the end of the string have all been cut rifled or button rifles barrels. The ones I've seen that gain velocity have been hammer forged. That is not an attempt to put forward any rule of thumb, just my observations.

 

[_Windrider_]

In regards to cold bore shots vs hot barrel shots, there are a few diferent things in play. Barrel lubricity is diferent for a cold bore shot because either the barrel is clean, or because the powder residue in the barrel has had time to absorb moisture. Whatever copper fouling might be present may have had time to oxidize, also affecting lubricity. I have found that If I run a bore snake or a dry brush followed by a dry patch, that my cold bore shots will most often, fall within the group.

Shots from a hot barrel will be affected by chamber heat if allowed to cook. I haven't been able to determine if this is due to the powder heating up, or if it is the heat affecting the primer mixture. The other thing is barrel expansion and contraction with heat.

The barrels I have seen that lose velocity toward the end of the string have all been cut rifled or button rifles barrels. The ones I've seen that gain velocity have been hammer forged. That is not an attempt to put forward any rule of thumb, just my observations.

So you believe that cold bore is real then?

 

[Rerun7]

Just went back and looked at some of my old shot string data. I’ve personally noticed no significant difference when shooting at pace. I’ll fire 10 rounds in under 2 minutes and haven’t seen any trending difference in velocity.

I’m sure it would happen if you let the barrel get really hot or let rounds cook before shooting but you aren’t going to throw off your measurements by not waiting 5 min between rounds.

Don’t over think it.

 

[Deleted member 113831]

So you believe that cold bore is real then?

Kind of, it's a qualified yes under the circumstances I posted above. I do think that the cold shooter syndrome has something to do with it.

If a shooter was to shoot the rifle one day, and then go out for a cold bore shot the next day, then none of the effects of changed bore lubricity would likely be present. It would probably be all the shooter.... if that shooter didn't take time to warm up with some dry fire drills .

But, put that rifle in the closet or a safe for a month or two, and that could be a different set of circumstances.

 

[_Windrider_]

Kind of, it's a qualified yes under the circumstances I posted above. I do think that the cold shooter syndrome has something to do with it.

If a shooter was to shoot the rifle one day, and then go out for a cold bore shot the next day, then none of the effects of changed bore lubricity would likely be present. It would probably be all the shooter.... if that shooter didn't take time to warm up with some dry fire drills .

But, put that rifle in the closet or a safe for a month or two, and that could be a different set of circumstances.

I can agree with that.