Higher velocities with shorter barrel?

[NYDAN]

Last Memorial Day weekend I stumbled upon a left handed CRF Win M70 Classic which I acquired. See viewtopic.php?f=9&t=39531.

The trigger was pretty bad on it and it wouldn't adjust properly so took it to my local gunsmith to have him fix the trigger which he did. The trigger worked great afterward. He advised me though that the "hot glue" bedding should be replaced, that there was roughness in the bore at the muzzle end, and the crown was off center. He suggested that I have him properly bed the action, free float the barrel, cut the end of the barrel to remove the roughness, and re-crown the barrel. However, he didn't have the time to complete the work before I needed the rifle for the fall hunting season.

I didn't get consistent accuracy with the original bedding job and resorted to installing a temporary pressure point between to forearm and barrel to get some reproducible results for hunting season.

After hunting season I took the rifle back to the gunsmith and asked to perform the work he suggested. I got the rifle back and was amazed at the improvement in accuracy of factory loads. My handloads still had the same accuracy as before.

I expected the velocities to be lower due to the shortened barrel and they were for the factory ammo. However, my reloads were faster in the shortened barrel.

Below are the results:

Load                     Velocity with 26” barrel    Velocity with 24.4” barrel    Difference  
Factory Rem 150 CL                    3100 fps                3065 fps                    35 fps less
Factory Win 150 PP                     3118 fps                3037 fps                    81 fps less
Factory Nos 140 AB                     3153 fps                3107 fps                    46 fps less
Factory Nos 168 ABLR                  2912 fps                2870 fps                    42 fps less

Reload-150 BT, R-23 (10/02/19)       2983 fps             2993 fps                   10 fps faster
Reload-150 BT, R-23 (02/21/20)                                 3010 fps                   27 fps faster

At first I assumed that the cartridges loaded on 10/02/19 had some molecular welding between the inside of the case neck and the bullet. So, I loaded more cartridges on 02/21/20 and shot those the same day. Those velocities were even faster.

The hand load is:

New Norma Brass
GM 215M
65.7 gr. Reloder 23
150 gr. BT
BTO 2.744" - .040" jump to lands
OAL 3.312"

Can anyone offer an opinion as to why the factory loads show a lower velocity with the shorter barrel whereas the hand loads show a faster velocity with the shorter barrel?

Thank you for your thoughts. Dan

 

[DrMike]

Likely the difference you observe is due to the propellants used. The factory loads were using non-canister propellants that are not available to the hand loader. However, these propellants are chosen on several criteria, including stability under a wide variety of conditions. The difference in velocities suggests that maximum velocities are being reach with the longer barrel. Perhaps combustion is not complete at twenty-four inches, whereas it is complete, or nearly complete, at twenty-six inches. The RL23 you used is showing that you have reached maximum velocity, so a difference of a couple of inches won't make much difference. In fact, it is likely that you would reach a similar velocity with a twenty-two inch barrel. Consequently, these thoughts are worth roughly what you paid for them, Dan.

 

[SJB358]

I’m with Mike on the propellants used for the loads and the difference is very small. I wished I knew why your loads sped up some but I’ll take free speed with a shorter barrel anyday. 24” and a 7 Rem are just about danged perfect. Looking forward to what it does with some more load work Dan.

 

[mjcmichigan]

Just restating Dr. Mikes point. If max velocity is reached 2” before then end of the barrel... the last two inches is pure drag... hence lopping off two inches... the bullet is not slowed down from the peak yet.

Apologies for restating the point... it had never occurred to me before.. so yet again learning stuff on the forum...


Sent from my iPad using Tapatalk

 

[Blkram]

I too agree on the propellant factor.
Two other considerations could be 1) that removing the roughness at the end of the barrel removed more drag (friction) on the bullet and caused the small increase in speed, and 2) perhaps the uneven crown was also producing a small amount of drag and yaw in your bullet, which may also produce slightly less velocity at the chronograph. Could not say for certain...but may be a factor

 

[gbflyer]

I’m still shedding tears that your smith talked you into cutting it. Blasphemy![emoji3]. This isn’t mine. I’d be tempted though to grab it just in case you want to return your rifle to collector status:


https://www.gunbroker.com/item/858204768

 

[NYDAN]

Guys, thank you for your thoughts. I appreciate you taking the time to respond. After reading your responses I went back to the QuickLOAD projection for this load and saw that it predicted 100% combustion at about 12". The pressure curve and the velocity curve both really flatten out after about 20". So your explanations seem reasonable to me.

gbflyer, thanks for the heads up about the barrel. My primary interest is in a good performing hunting rifle rather than a collectors item. But, I will sleep on it overnight and see how I feel about getting the barrel when I am rested. It has been a long day of wedding planning. :grin:

Dan