seating depth 308 palma

bulldog284

bulldog284

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i am looking for some advice on the seating depth for my 308 palma.i just got done building.i have tryed 4 different types of bullets.sierra 155 palma#2155and the #2156 nosler 155 match and the 155 scenars all bullets were just touching the lands.the load was 46.0g of varget cci br2 primers.

i have had fair results with the noslers and the sierras 2155 half minute of angle at 200 yards for testing.but when it comes to the scenars and the 2156 things open up to 1 minute plus not good.i have been told to either jump them or jam them???

if i was to jump them were should i start how many thou off the lands???not to interested in jamming because if they call a seize fire i am affraid the bullet will get stuck and i will have a mess.what seems to be the average jump to the lands or were to start?? thanks :runaway:
 
What chamber was your rifle made with? (SAAMI .308 Win, or a match reamer of some sort? Do you know at what C.O.A.L. a Sierra #2155 will touch the lands?)

The #2155 Sierra is a pretty tolerant bullet. Loaded to standard magazine length (2.800"), it shoot well out of most rifles.

The Lapua 155 Scenara (GB491) is a wonderful bullet, but it can be quite tricky to get it to shoot well; it can be quite sensitive to seating depth, as well as how hot the load is. I have found that it can be quite inaccurate with an overly-hot load. It took me years before I was able to find a full-power load that shot well and uniformly (I ended up using Lapua brass, seatignt the bullets about 10 tou clear of the lands, and enough Varget to give me 2950-ish in a 30" barrel).

I haven't used the #2156 Sierra, but everything I've read about it suggests to me that it is a wonderful bullet.

What overall lengths did you use on the ammo that you have fired so far? What brass did you use, and what velocities were you getting?
 
my overall lenght i dont messure i messure from the ogive and that gives me 2.116 with a sierra 2155#.my chamber is a 308 palma chamber and with that lenght i am just touching the lands,its seems to shot that bullet ok half minute.it is when i load the scenars and the new sierra that things open up.

i am using lapua cases 46g of varget and cci primers.i started with 45g and then 45.5g.and now 46g my next loadings i am going to try 10 thou of the lands and see what happens.
 
(Measuring bullet seating depth relative to the ogive is a superior way to record data for your own use in making precision rifle ammo. But it doesn't "travel well" to other loaders, e.g. what if I don't own a bullet comparator, or if I have a different brand than yours? Measuring C.O.A.L. is somewhat crude and nonrepeatable, especially with Sierra's irregular tips, but at least it is a universal "lowest common denominator" sort of measurement....)

I'm not sure what a ".308 Palma" chamber is, exactly; it's a relatively generic term. For example, one common reamer you see used a fair bit is the "'95 Warner Palma". which is an extremely short-throated chamber (IIRC .030" or .035" shorter than my JGS .308 Obermayer). But there is also a NZ 97 Palma reamer, and I suspect quite a number of others too.

Unless you tell me otherwise, let me assume that your chamber is a 95 Warner Palma, which was made for factory-loaded ammo using Sierra 155s (#2155) seated to "no more than 2.800"" (which in practice means that they were seated ten to fifteen thou short than that). This is why the throat is _so_ short, it's a case of designing a chamber to fit a piece of ammo, instead of the other way around.

Really short chambers like this work quite well with Sierra #2155s (as you yourself are seeing), and also apparently quite well too (i.e. I hear this, but I haven't done this myself) with the #2156, and the Berger 155.5 Fullbore bullet.

But, really short throated chambers are quite mismatched to the 155 Lapua Scenar; to get the same ogive:lands relationship, a very large portion of the 155 Scenar ends up being pushed deep into the case, taking up a great deal of powder room. Additionally, the bearing surface of a 155 Scenar is much longer than a #2155 Sierrs. Interestingly though, the ogive/nose relationship between the #2155 Sierra and the 155 Scenar is quite similar - you'll end up with very similar COAL measurements (from memory, I think the 155 Scenar ends up being .020" or .030" longer than the #2155 Sierra).

If you have a known good load for a #2155 Sierra, I would suggest you will find that the 155 Scenar will probably shoot well if you use about 1.0-1.5 grains less powder than you would for the Sierra.

My recommendation: I would suggest that if you have a short throated chamber, that you try to find a good load for the #2156 Sierra, or the Berger 155.5 Fullbore, rather than using the 155 Lapua.
 
as far as my chamber goes i am unsure of witch palma chamber it is but i do no it was made to shoot the 155 only.
i measured the coal with a loaded round tonight the numbers all are just touching the lands,for the lapua it was 2.780 and for the sierra 2156# it was 2.860.
why would you recommend not shotting the scenars and shotting the 2156# or the bergers?if the scenars have a higher bc.??and how far of the lands should i seat them? and the next question is were do you buy the bergers here in canada.

Darin
 
Darin, for what it's worth, I built an F/Tr rifle w/ a long throat specifically to shoot the 155 lapua- and couldn't get it to shoot well. I swapped in the 155.5 berger and have had a much easier time finding a load. The other good news is the 155.5 is ballistically equivilent or maybe slightly better than the lapua. This is not to say that you should give up on the lapua- just that it will take some tinkering, as you have seen.
 
bulldog284 said:
as far as my chamber goes i am unsure of witch palma chamber it is but i do no it was made to shoot the 155 only.
i measured the coal with a loaded round tonight the numbers all are just touching the lands,for the lapua it was 2.780 and for the sierra 2156# it was 2.860.
Click to expand...

OK, that's a very short throat; sounds exactly like a 95 Warner Palma, and if not that one, another chamber built along very similar lines. (And just to be clear: there's nothing "wrong" with that at all; that chamber was specifically designed to shoot the #2155 Sierra, the original "Palma bullet", as well as possible, when loaded to magazine length).

why would you recommend not shotting the scenars and shotting the 2156# or the bergers?if the scenars have a higher bc.??and how far of the lands should i seat them? and the next question is were do you buy the bergers here in canada.

Darin
Click to expand...

I don't know where to get Bergers in Canada (though I seem to recall some one of the Western sponsors of gunnutz might carry them?).

I suggested shooting the other bullets rather than the Lapua, only because of the short throat in your rifle. The 155 Scenar, the 155.5 Berger, and the #2156 Sierra, are all first-rate, extremely high performance bullets. Any one of them is fully up to the task of winning a world championship match. To a first approximation, they all have the same bc (basically, as high as has been achieved to date).

Which one of them you choose to shoot should be based on factors other than bc. For example: which ones can you get, how much do they cost, which one have you got the best load for, etc.

You should put one of each of these bullets side by side. You'll probably see that they are all similarly long (generally, the longer the bullet the higher the bc, and the lower the wind drift). You'll also probably see that the full diameter portion of the bullet (the shank) is about 80 thou further forward on the #2156 than on the Scenar - this is why the overall length of your ammunition, when loaded to touch the lands, of the Scenar ends up being so much shorter than the #2156. If you align the bullets at the front of their shank, you'll see that the base of the Scenar ends up being pushed much further into the case than the #2156 Sierra. If you had an extremely long throated chamber, you might find that this was an important advantage of the Scenar - it would allow you to touch the lands, while still holding onto the bullet. But you have an extremely short throat, so you have the opposite consideration - reaching the lands is no problem, you want to avoid stuffing the bullet into the case too much. A bullet such as a #2155 Sierra, or a #2156 Sierra, or a 155.5 Berger, do this much better than a 155 Scenar.
 
I suggest you start your test at 20 thou off the rifling. This works well with all bullets I tested and best with severl. Of course, each rifle is different, but in the 11 rifles I tested, ALL shot well with 20 thou off. I was testing Lapua, Berger, Nosler and Sierra. The only one that was fussy ws the Lapua. it was much better at 20 thou off than it was close. Velocities ran 2900 to 3050. 30" and 32" barrels. Powders were RL15, varget and N150.
 
Loads were developed for each rifle. Preferances were all over the place. The only thing I found was that 20 thou off worked well in everything. I used that as I tried various powder charges in 0.5 gr increments. Once I had the powder sharge that produced nice round 10 shot groups, i could try seating longer to see if it helped. Seldom improved.
 
ganderrite so you are saying no matter witch bullet i am going to try i should start 10 thou of the lands?? and go from there. i will try that and post some results. thank you Darin
 
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