Green Dot in .45 Colt effected by level of crimp?

[Southern Shooter]

I am just starting to explore using Green Dot in .45 Colt. Today, I loaded 7 grains under a 255 grain boolit. Out of my 5 1/2" Uberti velocities were in the very low 800's FPS. From reading of other's experiences with Green Dot and .45 caliber boolits of this weight I predicted hitting around 900 FPS from this gun.

My question is: Using Green Dot, does a weak/light crimp cause any issues like loss of velocity or erratic velocity?

Thanks

 

[243winxb]

Powder position may cause erratic velocity. Low velocity, compared to other gun, may be caused by a large gap between barrel & cylinder.

 

[g.willikers]

Were the velocities from the same loaded batch consistent and just less than expected, or erratic?
Those are two very different situations to ponder.

 

[the led farmer]

...From reading of other's experiences with Green Dot and .45 caliber boolits of this weight I predicted hitting around 900 FPS from this gun.

...

Are you talking about the boolits in general or fired from the same gun? Different guns, different velocities.

To answer your question directly in Modern Reloading Richard Lee references George Frosts book Ammunition Making who talks about "bullet pull" (the amount of force needed to pull the bullet from or push the bullet into the case) has an effect on, amongst other things velocity. So, the stronger the crimp, the more bullet pull, the more pressure and velocity is achieved. So yes crimp can affect velocity according to Lee and Frost. Makes sense to me

 

[SHR970]

Yes it can make a difference; the slower the powder the more the difference.

Your expected vs. actual velocities is much more likely a function of the gun it was fired out of and / or differences in components used.

 

[black mamba]

Crimp should only make a minor difference with a powder as fast as Green Dot. Oversized chambers, forcing cones and barrel-cylinder gap would be more likely culprits for lost velocity.

 

[rclark]

Crimp should only make a minor difference with a powder as fast as Green Dot.

Agree. My crimp is just a little crimp. Lip is just bent a bit into the crimp groove.

5200ft ASL

7.0g Green Dot, 250g RNFP, CCI-300, 917fps, 40ES, 5 1/2" barrel.
7.0g Green Dot, 255g _SWC, CCI-300, 894gps, 73ES, 5 1/2" barrel.

I would think you should be hitting at least 850 or so fps ... But velocity is affected by temp, altitude, cylinder/barrel gap, cylinder throats, forcing cone, barrel diameter... So many different things. Each gun is different. That is why I tend to do 'all' my chrono testing for a caliber with one particular revolver, so my resulting table is consistent.

So, the stronger the crimp, the more bullet pull, the more pressure and velocity is achieved.

I should do a test ... but my mind, the crimp really it there to prevent bullet creep from recoil. The high pressure that is generated, makes the crimp just a very minor iritatation when bullet leaves the case. For slow powders, it does allow the pressure to build a 'tiny' bit more before pushing the bullet past the crimp. The case tention on the bullet is much more important is my understanding.