Bullet Knurling First Impression

[ET1]

When this concept was first introduced to me I thought this was a real off the wall approach. Here I envisioned erratic sabot release that should throw groups all over the place and erratic bullet flight.
Boy was I proven wrong from my own target results and was really impressed.

So what is the purpose for bullet knurling?

1) To create a better gripping bond between copper jacket bullet and sabot. Smooth on smooth is not the best bond between the two.
2) Knurling bullets for increased diameter for better load resistance fit to bore.

When you examine the bullet and look at the area of bearing wall length (straight wall) that varies with bullet size and design that is all that grips the sabot and is utilized to spin that bullet at thousands of RPM for flight stabilization. Now if the bullet has good obturation qualities then the majority of the time this is not an issue. But take a poor obturation quality of an all copper bullet then this could be another story. To go a step further some copper bullets have cut grooves reducing the grip area even more.

If I had to choose one bullet for hunting all size game in North America the 275-XPB from Barnes would be my choice. For moose hunting this would be IMO an optimum bullet to use. Now try and get a stabilized flight for accuracy without knurling.
Yes this bullet is pricey by most standards but what price do you put on taking down a moose more efficiently?
Yes knurling is another step that can be a little aggravating that most shooters don’t want to be bothered with and understandable. If you want ensure sabot grip to bullet or in some cases need a tighter fitting load by slightly increasing the bullet diameter knurling is a viable option that does not appear to hinder accuracy.

For light knurling a coarse file rolled over the bullet is all that is needed. Of course downward pressure on the file is also required. The file doesn’t simply dig quickly into the copper so fear of quickly over knurling is not warranted.

The value of knurling to me is that every bullet I use or try to use gets knurled so possible slippage is never considered when my groups are not responding to my expectations.
This thread is intended to reveal possible improvement with accuracy when shooting sabot loads. Whether you use this information or not is up to you and is an open option if you are looking for a better possible or dependable response from your sabot/bullet loads.

knurled bullets.jpg

Ed

[Big Jack]

Never played with that but will give it a go!

[ET1]

Big Jack

What I found interesting about sabots is they seem to work best for bullet grip in a certain temperature range. Temperature alters the physical property of the plastic used for the sabot. Too hot or too cold appears to alter the grip quality as well. Add knurling to the bullet then the grip becomes a non issue.

Look forward to hearing of your observations when you try it.


Ed

[Big Jack]

Should be interesting.

Have you ever played with Duplex Sabots? PRbullet sells them.

I am am not talking about duplex loads - just to be clear.

[MikePal]

I case your wondering what Big Jack is talking about:

http://www.prbullet.com/duplex.htm

[bdog]

Hey Ed, when knurling your bullet, try laying one file flat on the bench and put the bullet on there and then the other file to roll on top and knurl, I had better results like that....

[ET1]

Big Jack

No I haven’t tried PR duplex sabots but have played with my own version of using Harvesters sabots in trying to get a 40cal 200gr bullet to fly decent. No luck once I got over 2100fps. Not finished experimenting with that load yet because I believe I was having a separation issue with inner sabot as velocity increased. MMP is experimenting with a new formulation that I hope they will adapt to their existing 40-50Cal sabot line as another option.

Trying to get around up to 2500fps with 200grFTX .451 bullet and shows better promise at the moment. I’m looking for a nasty varmint bullet load. A big issue here IMO is bore roughness for bullets with a shorter bearing surface for grip. Hopefully with the form of lapping I’ve tried will help in this area.

Bdog

Thank you for the considerate thought but I now have another method that is actually quite fast. The way you described your approach is the next best method to use especially if you want to add some diameter to the bullet. A single file is good for knurl impression but requires a lot more effort for bullet diameter increase.

Mike Pal

Again thanks for your thoughtfulness in trying to help with clarification.


Ed

[ET1]

Big jack

Went and dug into my files for duplexing sabots and came across them. Recording my endeavourers for future referencing makes life easier when I return to a given load. The picture below is trying to duplex sabots. The first thing I do is machine off the base cup of the inner sabot for fit into the outer sabot to prevent bullet drilling. Now I’ve tried keeping the longer petal length of the outer sabot and in the picture reduce the petal length of the outer sabot to see what would happen. Here I would locate the outer sabot after the shot but never have found the inner sabot after a shot. This is with loads exceeding 2100fps.

The only conclusion I can at the moment come to is the outer sabot base cup when flared is creating a drag resistance that assists sabot release. I tried different glues to see if I could bond the 2-sabots but this type of plastic will lend itself to gluing. The only real bond would be to use heat to fuse the 2-plastics together. And yes I may have found a way to fuse the petal tips together but around this time the 200FTX .451” came along and my focus has been with this load for now. Eventually I will return to sabot duplex just to see if what I done works or not.

Man to get a 40cal bullet accurately whizzing at 2500fps out of a 50cal muzzle loader with a higher BC would be something.
Anyway I’m also working on another project for increasing the efficiency of 209 ignition and further reducing blowback along the 209 primer. In the primer pocket I’m installing a shelf for the primer to butt against for additional sealing from blowback gases. This is not an original idea from me but from others who are getting promising results.

Sabot Base Trimming Tool B.jpg40-50 200SW Load.A.jpg


Ed

[Pat32rf]

What about the adapters to use large rifle primers instead of 209s? Or are they designed more for speed and ease than accuracy?

[ET1]

Pat32rf

The adapters also commonly referred to as modules for using rifle primers (Magnum preferably) is supposed to reduce blowback and provide a more consistent ignition that in turn is supposed to help accuracy and may increase speed somewhat. Never having used them I can’t vouch for their claim but many others have reported good responses from using them.

I enjoy the simplicity of installing a 209 primer and the weakness is the sides for blowback. I agree that blowback can cause loss of thermal energy delivery to the powder load. I’m currently working on a mod to see if better control of blowback can be achieved with 209-primer. Yes head spacing can help with blowback but does not seem to totally eliminate it. Also some of the blowback issue can result from returning bore pressure.

By installing a shelf in the BP for the bottom of the 209-primer to butt against in turn reducing access to the primer sides will hopefully reduce blowback even more or may eliminate it. Currently my setup will reduce the primer (W209) length by .002” when cycled in this system so there is compression for seating primer involved.

Can’t report much at the moment as this is in the R&D stage for testing to be started this year.

Primer Shelf.jpg


Ed