O A L AI .308 chamber

[ColinR]

Something I have been meaning to do for a while, measuring* distance to lands on AI AWM .308 using Lapua Lock Base 170 grain FMJ. Touching the lands gives an OAL of 2.860" against the Vihtavouri Guide suggesting 2.795" OAL - a difference of 0.065". Given that I might start reloading at 0.020" off the lands this still gives me an actual OAL of 2.840" against the Vit rec of 2.795" or 0.045" longer. I usually use 45.5 of N140 behind the 170 Lock Base set to an OAL of 2.795" with good results, but with the bullet jumping, I now realise, some 0.065" before it meets the lands.

A) Are these the sort of measurements one would expect from an AI AWM chamber?

B) Am I likely to encounter any problems increasing the OAL by >0.045" from recommended -- still short enough to fit magazine -- need to increase load?

C) If left as is is this excessive jump likely to increase throat erosion? (Maybe a reason to explore further)

D) Is it a waste of time developing this load further given it shoots OK at the moment? (silly question to which I should know the answer, well it seems obvious except for C) above)

* Using Hornady tool

Asking a lot, but I have no experience on this point, so thanks for any help forthcoming.

[20series]

the 2.795 is the minimum length, you should be fine doing loads anything over that.

My 200 gn SMK Remmy oal is loaded to 2.870 to the tip or 2.286 to the ogive which is the better way to measure for accuracy as the tips can vary

Alan

[ColinR]

the 2.795 is the minimum length, you should be fine doing loads anything over that.

My 200 gn SMK Remmy oal is loaded to 2.870 to the tip or 2.286 to the ogive which is the better way to measure for accuracy as the tips can vary

Alan

With the 170 Lock Base it measures 2.257" to the ogive.

[ovenpaa]

Mine was 2.865" at new with 170 LB's so very similar. The AI is chambered in 7,62x51 at the more open end of the specification to allow for the vagaries of assorted military ammunition manufacturers, I built to 2.850" and had fine results to 1000 and it worked to 1200 on a warm day.

I was running a warmer load than you with N140 and eventually moved to N540. The 170LB is as good a bullet as I have ever shot in my AI but had one odd problem I could not quite overcome. Apparently they work well with N550 behind them as well.

You are way within magazine OAL which (from memory) is 2.990" so a nominal 3.000" so you can build at whatever length suits you.

[ColinR]

Mine was 2.865" at new with 170 LB's so very similar. The AI is chambered in 7,62x51 at the more open end of the specification to allow for the vagaries of assorted military ammunition manufacturers, I built to 2.850" and had fine results to 1000 and it worked to 1200 on a warm day.

I was running a warmer load than you with N140 and eventually moved to N540. The 170LB is as good a bullet as I have ever shot in my AI but had one odd problem I could not quite overcome. Apparently they work well with N550 behind them as well.

You are way within magazine OAL which (from memory) is 2.990" so a nominal 3.000" so you can build at whatever length suits you.

Thanks for the confirmation Dave. I will try setting bullet at OAL of 2.840" which will give a nominal 0.020" standoff from the lands and see where that takes me. I am presuming setting the bullet out an extra 0.045" will have some effect on pressure so I'll start at 45.7 grains of N140 if you think that is reasonable. While on the subject I have been a loyal user of Vihtavouri powders since my pistol days, but like you I do find they fluctuate considerably with ambient temperature in rifles. I use N140 (.308) and N165 (.338LM) - are the N500 series more temperature tolerant, or maybe a complete change of manufacturer - am I right in thinking Hodgdon powders are more temperature stable? Incidentally what was the problem you encountered with Lock Base? My current loading of 45.5 N140 170 LB with a jump to the lands of .065" works acceptably well at 900m, so it will be interesting to see how these new dimensions affect accuracy

[ovenpaa]

I find Viht powder fairly stable regardless of temperatures the limitation is my S&B which goes to 13.1 Mil and I needed 13.4 on a cold day. .020" off the lands will be a good starting point, the 500 series does seem slightly happier in the cold but the issue is the ambient not the powder.

Hodgdon certainly does not seem any better and also seems to vary from lot to lot, Christel uses H4831SC which is in the N560 region, she buys load of tubs of the stuff, gets a big mixing bowl and stirs the whole lot together then sticks it back in the tubs.

The problem with the 170LB was flyers, I could hold a very good group with 19 shots, the 20th would be a flyer and could be in any direction, I never did get to the bottom of it and eventually moved to 155 Scenar's when 170's became hard to find. I would happily go back to the Lockbase though, regardless of the odd flyer issue.

[ColinR]

The problem with the 170LB was flyers, I could hold a very good group with 19 shots, the 20th would be a flyer and could be in any direction, I never did get to the bottom of it and eventually moved to 155 Scenar's when 170's became hard to find. I would happily go back to the Lockbase though, regardless of the odd flyer issue.

It maybe something of nothing but I had 200 x 250 Lock Base .338 replaced due to polishing media contamination in the base of the bullet. Small flakes of wood and polishing compound stuck over the lead. Lapua relaced them all and left me with the faulty bullets which were remedied over the course of one evening with a dental pick and eyeglass - not something I expect to do at >£65 per 100, hence the complaint. They shot OK. I can't say I have any experience of shooting them with the contamination so can't comment as to any effect this might of had. Lapua said it was a known problem but that they had no current solution due to the type of polishing media they were using. I mention this as I just checked through a box of 170 LB's and found a few with the same problem. The 250's all had the problem but because the hole over the lead is smaller on the 170's only a few seemed to suffer this problem. Could it be the cause of your flyers??

[ovenpaa]

I also saw this with the 170's and also picked the bits out, I still had very occasional flyers. I spent quite a while measuring and weighing the bullets and never did spot anything untoward, in fact the all came up very well so I stopped checking and had a couple more flyers. :lol: