Same ammo, same barrel length, same height above bore, but different DOPE?

[Sheriff_Will_Teasle]

Rifle #1 is a Vudoo, 18" Kukri, shooting SK at 1080 FPS (ES 29 / SD 8). Scope height above bore is 2.4". DOPE @ 100 yards is 1.5 and at 200 yards is 6.6.

Rifle #2 is a Savage B-22, also 18" barrel, same height above bore. I don't have chrono data on the muzzle velocity (I'll get it at some point). But with the exact same ammo I see 1.9 @ 100 and 7.4 @ 200.

Messing around with the Applied Ballistics app it looks like an average MV of 1050 FPS would match up with the Savage DOPE using the same BC (G1 0.15).

So my question is - is this possible? Can the same ammo going down the same barrel length drop in MV by 30 FPS (4x the SD?)?

At some point in the next few weeks I will get out to the range and test with a Labradar, but I'm looking at 4 days of snow next week so it may not be real soon. In the meantime, this discrepancy is bothering me and I wonder what the hive has the say about it....is there anyway to explain this with barrel harmonics (I don't think so because the difference in DOPE is non-linear)?

 

[1911JMB]

Yup some barrels are faster than others

 

[sheepdog697]

I don’t have much experience with rimfire MV. I can tell you that my factory tikka 24” 6.5 is about 100fps slower than my buddies 6.5 24” custom with bartlein barrel. Same ammunition. People have been known to change factory barrels on the tikkas just because they are slow even though accuracy is great. Frank talks about this in a couple videos.

 

[Precision Underground]

I’d be much more surprised if the dope for two different barrels matched up.

 

[acudaowner]

was the wind at your back ? your range haunted ? a pretty girl at the range ?

 

[justin amateur]

No two boxes of rimfire will produce the same average muzzle velocities.
Not even from the same brick. I use a ballistic chronograph almost every trip to the range.
It's the only way I have to ensure those high and low strays aren't my fault.
30 fps difference is no surprise with 22lr from different boxes of cartridges with the same lot number.
Often not even from the same box.
I've seen it from SK, Eley, Lapua and RWS.

It's rimfire which means it's damn difficult to manufacture those cartridges
so that they are uniform in quality and performance.

 

[grauhanen]

So my question is - is this possible? Can the same ammo going down the same barrel length drop in MV by 30 FPS (4x the SD?)?

At some point in the next few weeks I will get out to the range and test with a Labradar, but I'm looking at 4 days of snow next week so it may not be real soon. In the meantime, this discrepancy is bothering me and I wonder what the hive has the say about it....is there anyway to explain this with barrel harmonics (I don't think so because the difference in DOPE is non-linear)?

There are two important factors among those that explain what's observed here. One concerns the ammo, the other the barrels.

Despite using ammo from the same lot It's not possible to shoot "the same ammo" (the OP's words) twice. As justin amateur notes about .22LR ammo in general, it should be considered that SK ammo, while a good value, may not have the consistency from box to box within the same lot to draw reliable conclusions when comparing one relatively small sample size with another.

More important perhaps is assumption that barrels that are the same length should shoot similar ammo with similar MV and ES. As noted above by 1911JMB, "some barrels are faster than others" -- even barrels of the same length. Even when barrel diameter is the same, if bore length is what two barrels have in common, the bores themselves may be different in other ways. These include bore diameter and diameter consistency through its length, bore concentricity, and rifling characteristics. Just because two apples look similar it doesn't mean they'll taste the same.

Related to what's noted above, barrel diameter in general may also play a role in obtaining different results. Barrels of different diameter will experience vibrations differently with each shot and this can have discernable results in different POI for each barrel.