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For the first time I've measured my OAL (using Ovenpaa's rather smart collimator and OAL gauge) and come out with an average of 82.13 millimeters. I've heard differing opinions on how much free flight (?) I want before my bullet engages the rifling. What would you recommend?

My load is .308 using SMK 155 (2155).

TIA

Watcher

Watcher,

I will throw in a response and see if my answer gets corrected!

The gap that you refer to is what is known as "the jump". Where the bullet touches the rifling that is the lands.
You will have read of shooters talking about their ammunition having or performing best with a lot of jump, backed of a specific amount, touching the lands and in some cases rammed into the lands.
At either ends of those options you will find that you are increasing the amount of pressure expeditiously and you should not assume that your normal load will be safe as it almost will not.

So what is right for you, your load, your case your chamber your barrel etc?
Well no two rifles are made the same not even the mass produced Remington 700 and as a rifle gets shot the lands will be worn away and the jump will get longer. So it is a case of making up a number of experimental rounds getting to a reasonably safe distance, many chose 100 yards and shooting to see what rounds perform best. The best rounds will probably be a combination of small groups and if you have a chronograph a average spread which is within your acceptable standards. You may for example be wanting to shoot at 1000 yards with this set up in which case you will bee looking for around 3000fps. No good having a superb group at 100 yards but not having enough fps to get you on to the target at your max distance.

From my experience with the 155 SMK's I gave them a small jump, so I could get them in a magazine (another consideration) but the jump was not massive.

Good luck with finding a load that works well in your rifle, we will all want to know how you get on,

DM

I would suggest an initial jump of around 0,30mm to 0,38mm (.012" to .015") However you could start at 0,50mm and then just move forward in more steps, I would not get any closer than 0,15mm/0.006" Are you magazine or single feeding the rounds? If magazine fed you would need to build to suit the magazine OAL

ovenpaa wrote:I would not get any closer than 0,15mm/0.006"

Any reason why, or just personal preference? Only asking as I'm sure my Stoney Point OAL manual recommends somewhere between two and four thou off the rifling is the optimum......

That said, Roy Weatherby, of the rifle and propriatory cartridge fame, was a great proponent of lots of free bore!

Sim G wrote:Any reason why, or just personal preference? Only asking as I'm sure my Stoney Point OAL manual recommends somewhere between two and four thou off the rifling is the optimum......

That said, Roy Weatherby, of the rifle and propriatory cartridge fame, was a great proponent of lots of free bore!

Yes you can get potentially greater accuracy with minimal jump however it is also very easy to end up with a jam condition, higher chamber pressure and vertical dispersion so it is sometimes better to stop back a couple more thou.

ovenpaa wrote:I would suggest an initial jump of around 0,30mm to 0,38mm (.012" to .015") However you could start at 0,50mm and then just move forward in more steps, I would not get any closer than 0,15mm/0.006" Are you magazine or single feeding the rounds? If magazine fed you would need to build to suit the magazine OAL

Single feed in a Swing clone (Angel M80).

I would work around the 0,30mm mark to start off with.

Thanks guys.

When i started reloading i smoke tested my rounds and made them so they were just a little off the lands. but i found that on my .30-06 there was too little shank in the neck. (said the actress...). One day, early in my stalking career a round got the bullet pulled out when i openned the rifle because of the inaccuracy of my method.

the rifle, and that load was shooting .5 moa and i thought it was great...

but after that bullet pulling episode i thought "well thats the end of that days shooting."

so i went home and pressed them back to what it said in the book, I think it was 3.25 in.

next time i tried to shoot a group, with a bit of trepidation, i was very surprised. it made no difference to the accuracy over 100 yards.
In my 7mm08 i use hornady bulltes and they have quite an angular ogive. if they are loaded to the right length for accuracy they are too long for the magazine. But this is a stalking rifle only. I can stomach a very slight loss of accuracy.

The key, i think, is trial and error.

steve