Lapua v RWS case Conundrum

[Brillo]

In my .308 rifle I use mostly Lapua cases, but I recently managed to get some 'once-fired' RWS cases.. The RWS cases require a 0.334" bushing to obtain the same neck tension that I get from a 0.336" bushing with the Lapua cases. Also they weigh a lot less than the Lapuas'. From this it is reasonable to assume that the case capacity of the RWS is greater than that of the RWS case.

What I don't understand is when I compare exactly the same loads, regardless of powder or bullet used, (e.g. 46.9 grain of TR 140 and 155 gr SMK Palma) I consistently get higher MVs from the RWS case, sometimes by as much as 50 fps.

[Swamp Donkey]

Could be a 'lighter' make up of the brass alloy causing less weight. I'd do a water test to see capacity personally.

But, interesting. Post your results

 

Pete

[jcampbellsmith]

Brillo

 

It's an interesting question. You don't say what kind of resizing you are doing on your brass. I take the view that I need to work up a load for each brand of brass that I use and I just accept that they will perform differently.

 

Regards

 

JCS

[Scotch_egg]

I found I get better result using once fired RWS brass than the lapua.

[GeorgeB]

I also found RWS 308 cases to give higher MVs.

 

I put it down to shorter primer flame path, due to shallower base and/or chamfered flash-hole.

 

I ended up using the mildest primers I could find, to avoid pressure spikes.

[Brillo]

Brillo

 

It's an interesting question. You don't say what kind of resizing you are doing on your brass. I take the view that I need to work up a load for each brand of brass that I use and I just accept that they will perform differently.

 

Regards

 

JCS

Regardless of whether I full-length or neck size the RWS cases give me higher velocities than Lapua cases.

[gbal]

I also found RWS 308 cases to give higher MVs.

 

I put it down to shorter primer flame path, due to shallower base and/or chamfered flash-hole.

 

I ended up using the mildest primers I could find, to avoid pressure spikes.

If the cases do differ in capacity,would there be differences in either pressures,or total burn times/rates,(including these primer effects) which might affect velocity?

Gbal

[Laurie]

Interesting! The batches of Lapua and ex NRA RWS brass I have produce similar water overflow capacities in my rifles (~56gn), so I'd expect similar pressures and MVs - but have never done a side by side comparison. GeorgeB's point is an interesting one I wouldn't normally have considered, although I know that that the Lapua Palma case's use of both the small primer and (just as important) the small 1.5mm dia. flash-hole has a major effect on charge burn and pressure build-up. In this case, the ignition effect reduces MVs and around an extra 0.5gn of powder has to be used.

 

Different levels of neck tension can also affect MVs, both in absolute terms and in the size of the velocity spread. As the two have pretty similar neck thickness, this would appear to be a brass mix and / or annealing issue. Each maker uses its own 'recipe' for cartridge brass. so there are variations in basic hardness and ductability (if there is such a word!).