It is time to start looking at getting some rounds through the recently received Steyr SSG 69 P1. In theory with a 1:12 twist and a 660mm/26″ it can cope with anything in the 145-175 grain range with ease and this very nicely brackets the available military ammunition plus civilian/law enforcement applications such as the 155, 168 and 175 grain bullets so quite a choice really.
I have opted for the Lapua B476 170 grain Lockbase. So why should I choose what is a reasonably old form bullet for such a rifle? Well that is quite easy to answer, the 170 Lockbase was the basis of the Lapua factory .308 HPS (High Performance Sniper) and the more recent .308 HPS Ti (Thermally Insensitive) and has been used to great effect in theatre and was a firm favourite in my AI when it was chambered in 7,62×51.
Lapua tell us The 170 Lockbase provides the shooter with an exceptionally flat trajectory, and extends the effective range of the 308 Win to near 300 Win Mag territory.
That sounds impressive and it certainly worked well before with a stout load of N540 so good enough reason to use it again. The only thing is the Steyr has a push fit barrel and I really do not want to start swapping out such a barrel as it involves machining out the old stub and then threading the inside of a hardened receiver, either that or sending it back to Steyr for a new barrel which is not going to be cheap. So my choice of powder will be TR140 and my plan is to emulate the later HPS Ti as best I can. Easier said than done, the new HPS has staked primers, a special undisclosed powder (The older HPS used Bofors powder) and was built to 2.795″. The 170 Lockbase has a G1 BC of 0.498 and a G7 BC of 0.249 so reasonable but not cutting edge. One thing in favour for the 170LB is it is formed from the front and the copper jacket finishes with a boat tail and a strange closing resulting in a hole with slightly raised edges, the purpose being to allow the air to bleed from the back of the bullet so reducing drag and aiding stability through the transitional stage.
Now the original factory 170 HPS had an MV of 2820fps (860m/s) and the newer 70 HPS Ti 2756fps (840m/s). I only have 10 Mils of elevation on my ZFM6x42Z so it is certainly not going to be a 1200 yard rifle however that 10 Mils on a warmish British day (60F/15C) is going to get me to the target with a MV of the later HPS Ti so that is it sorted, I will load with the goal of 840m/s. Now I know this does not mean I am building HPS Ti however this is England, temperatures are remarkably stable across the shooting year and I very much doubt I will see many days outside of 10C to 22C (50F to 72F) and at this range of temperatures the HPS Ti holds a very steady 840m/s
Now there is one minor dilemma, the OAL to the lands of the SSG 69 is 2.890″ and I previously built to 2.875″ so a 15 thou jump in the AI, if I am to build to 2.795″ I am closer to a 95 thou jump however this does mean I have lots of room to experiment with as I move towards the lands. Finally, the load of TR140, well the good news is 100% case capacity is virtually the exact load required to get me to the desired 2756 fps (840m/s) Coincidence or what? I will of course start a little bit lower and work the load to the desired point, across a chrono and looking for signs of over pressure. The SSG’s 6 lugs look OK however this is a used rifle, albeit a very clean used rifle so I have no real indication of the rifles previous life.
My predicted starting drops are as follows and I will be taking my Kestrel 4500AB along as well to run predictions on the day. Yes I know electronic aids are not allowed on the firing point in competitions however this is not a competition and I will take all the help I can find when shooting a new rifle for the first time.
Right so assuming this side of things goes according to plan the next job is to fit a new ballistic cam to suit the new loads performance. I rang Swarovski UK a couple of days ago to see if they could supply a bikini ‘scope cover to replace the awful BC’s that were fitted, the plan being to move on to a replacement cam. That was the plan however Swarovski UK denied any knowledge of the ZFM6x42Z, then told me it has been discontinued for years and they held no spares, however if they did find anything they would call me. CLUNK and the ‘phone went down at the other end. So let’s assume the UK are not going to have any cams. Of course I can always ring the Austrian office however I decided to drop the existing cam off and dimension it so I can build my own.
What I will do is build a couple of blanks and fit one to the elevation drum, add some masking tape or similar and then shoot as many distances as possible to set the datums, from this I can calculate the missing distances and engrave them along with a number to indicate the distance, once done I can anodise the cam and white in the numbers and the job is done. Why two cams? Well you just know what will happen if I only build the one…..
Help yourself to the dimensions if you fancy building your own blank ballistic cams 🙂