JAM FIT

[nell]

Im reloading a 6mmbr 1;12 twist barrel using 75gr v max, whilst searching for data JAM FIT keeps comming up as good OAL.

 

Any body familiar with this?? I use the split case neck method for finding a start point for OAL,is this what i imagine a JAM FIT to be??

 

If so, would you start with the normal 10% reduced load or less when working up a load, as i can only imagine excess pressure is going to be an issue.

 

Any info please

 

Nell

[Streeker59]

A jam fit is exactly what is says on the tin. Set your OAL longer than your chamber length so every round load is "jammed" into the lands..

The only problem with this method is that you get one pressure curve instead of two..

When a round is loaded "off" the lands and fired, there is an initial peak pressure curve to get the bullet to overcome the case neck tension.

As the bullet starts to move, the pressure drops slightly then rises sharply as the bullet enters the rifling and begins to bite into the lands.

 

This two pressure curve system is considered by some to be the preferred method of handloading.

The up side of "jamming" however is that the bullet is already aligned with the bore before the round is fired.

Having said that, "jamming" doesn't always offer the best accuracy.

Tangent ogive bullets prefer a jump whereas secant ogive bullets work well if jammed..

[garyw]

i "jump" the 75 vmax in my 6br by 0.005 with n135 when i use varget for the 75 i go to 0.008 doesnt sound a lot but both powders give same results with altered seating depth.

i would advise you not to "jam"bullets with the 6br only because my testing resulted in pressure with mild loads,but each gun is differant.

there are more folks on here who will help im sure.

gary

[nell]

A jam fit is exactly what is says on the tin. Set your OAL longer than your chamber length so every round load is "jammed" into the lands..

The only problem with this method is that you get one pressure curve instead of two..

When a round is loaded "off" the lands and fired, there is an initial peak pressure curve to get the bullet to overcome the case neck tension.

As the bullet starts to move, the pressure drops slightly then rises sharply as the bullet enters the rifling and begins to bite into the lands.

 

This two pressure curve system is considered by some to be the preferred method of handloading.

The up side of "jamming" however is that the bullet is already aligned with the bore before the round is fired.

Having said that, "jamming" doesn't always offer the best accuracy.

Tangent ogive bullets prefer a jump whereas secant ogive bullets work well if jammed..

 

Thanks for replies folks..

 

Could someone explain the difference between Tangent and Secant ogive bullets, and there significance??

 

Nell

[Streeker59]

Click on this Nell. It will tell you all you need to know but basically a bullet with a tangent ogive has a longer bearing surface. A secant ogive bullet has a shorter bearing surface. As such the secant bullet needs to be closer to the rifling to ensure accurate alignment with the bore.

 

http://accurateshooter.wordpress.com/2007/07/10/283/