Checked chamber lengh for my 1893 Spanish Mauser and got 3.165" to the lands...

To put it quite bluntly (without intending to be rude!) - Stop messing around with plinker bullets!

I copied this from a previous thread on the same problem (in a Mosin-Nagant 91/30 sniper):

".. A possible problem is that the throat of the rifle is deep enough that the boat tail is coming free of the cartridge neck before the bullet is engaging the lands. This can actually lead to a worse internal ballistic performance and group size than with flat-base projectiles, as the gas blow-by on the bullet can cause it to be engraved with a slight skew**. Check the absolute maximum OAL you can use (with the bullet touching the lands) and if, in this position, the start of the boat tail is out of the case, stick to flat-base. That is precisely the situation with my 91/30. So the direct answer to your question is: yes I have tried HPBT, but the flat-base round-nose or spitzer types fly better in my rifle!

*Get the Sierra book for detailed trajectory values.

**I suggest that you get out your pocket calculator and work out how much skew over the length of the parallel section of an HPBT corresponds to 1 MOA. It is very, very, small."

and next day:

"Ah well, since no-one else worked it out yet...
Using the old rule of thumb:

1 MOA corresponds to 1" at 100 yds.
so
1 MOA is 1/1000" at 1/10 yd or 3.6"
and over half an inch (the approx. length of the parallel section on an HPBT)
1MOA is 1/1000" divided by 3.6 x2 or 7.2

About 1.4 tenths of a thousandth of an inch. Small enough for you?

So now it is clear why the bench-rest boys measure things like bullet run-out in the case.
And why they try to get the "slop" of the neck in the chamber down to well-nigh zero.
And why neck sizing your fired case, leaving an ever-so-tiny unsized ring at the bottom of the neck that provides automatic centering of the neck in the chamber improves accuracy.

It's not just nit-picky fiddling around - it really can make a difference - if you and your rifle are good enough."

Use the longest bullets you can find for the 7x57. Flat base bullets. Something in the 160-180 grain range. For instance, the S&B "SPCE" bullets produce excellent results, as they have a long cylindrical section up to the step. They also cut very clean holes!

Bob has mentioned using flat base bullets which I will use. It's the obturation he was saying I need to make sure happens so the bullet actually goes into the grooves. Loading much longer and using the heavier bullets the rifle was designed for will probably help also. I bet I get at least fist sized groups next weekend. I'll report back after. I consider plinker bullets anythijng not expensive so I think flat base bullets don't have to be 50 cents each right? The plinkers I plan on getting next are 173g flat base bullets that one store said they should have in stock this week. I'll just be plinking out to 50-100 yards so not long range. I'm silly so I googled plinking and it actually has a Wiki page hehe geez.
"Plinking refers to informal target shooting done for pleasure, typically at non-standard targets such as road signs, tin cans, logs, bottles, or any other homemade or naturally occurring target"
Thanks Patrick, at least I know it's probably not a broken rifle.

"173g flat base bullets " - that's not plinkers, that's serious bullets. By plinkers I meant those lightweight things.

BTW, I have seen a diagram of what happens when a bullet is engraved with a skew. The trajectory becomes a spiraling spiral (or coiled coil) pattern, and it is absolutely impossible to predict where the point of impact will be in the spiral motion. So grouping is very, very bad and cannot be improved by fiddling with the load etc.

An unevenly worn muzzle or damaged bullet base has the same effect. In this case, the skew arises when the bullet receives an asymmetrical impetus on leaving the muzzle.

Yeah I call cheap bullets plinkers regardless of weight since plinker just means not serious target shooting vs match grade. All bullets are serious but match grade would be non plinking. I have some expensive 125g flat base match bullets for my 308 that are much much more serious than my and cheap 150 and 168 grain bulllets but sounds like you use weight to define plinking vs serious match stuff it's all good. I can't wait to shoot next week and especially if I get the 173g bullets. Thanks again Patrick

Some of my very best target load for 7x57mm military rifles were light loads with the very light bullets of 100 to 115 grains . PERIOD . I am talking about 1 to 1.5 inch 5-shot groups at 100 yards shot on demand during rifle matches . So do not tell me it does not work . I am talking about 1000's of test rounds through over 50 different 7mm rifles .

Thanks Bob. I'm sure the flat base and loading a little farther out will help. It's like you said I need to make sure the bullets obturate into the groves. I took a couple pics of the muzzle and the rifling looks fine so I'll try to post here. I was wondering if you would notice the previous posts with someone thinking things lighter than 173g is just a plinker regardless of quality, match grade, price, etc.. He probably just meant to say heavy which in my case doesn't matter as much as the flat base should help but the longer length I'm sure will help also in my rifle. I dropped a bullet into the chamber with the rifle pointing down and the bullet drops past where the neck ends. In your experience have your other rifles done that and still shot fine?
Someone on another forum told me about counter boring. I'll post the second picture also. One of the grooves seems to fade out which is probably causing some issues also. I'm asking my local gunsmith if he counterbores and how much. Video below does a good job explaining to me in case anyone else searches and finds this thread it could help them.
Military Surplus Firearm Collecting - Episode 1: Counterboring. What is it, why, and is it bad? - YouTube

Took better pics of the rifling at the end. Maybe they fade in the last couple millimeters so maybe not too bad to need counter boring?

All chambers have a throat so the bullet has some jump otherwise to engage the rifling I.E cammed into it by the bolt would cause a very dangerous pressure level. I would also check the crown of the barrel to ensure no damage has occurred bit hard to tell as your pics are pretty fuzzy.
If one land does end before the others then the bypass of gasses may cause a skew and upset the bullets flight, I gather the bedding is good and the H/S in spec sometimes it can be a very simple thing causing a group dispersion.

When you do get it sorted I found a ladder test is the quickest way to find a load that groups you can then work on getting it even tighter with development, hope you get it sorted I know what its like to own a shotgunning milsurp my sniper T was one such animal.

Thanks Cinders. I'll load up some 175g flat base tomorrow when they arrive in the mail. Between longer bullet, flat base, loading longer COL that will probably help. I might try the brass round headed bolt and lapping compound to clean up the end of the muzzle if there is any small burs I can't see. I bet the groups tighten up. I'll post pics

"but sounds like you use weight to define plinking vs serious match stuff it's all good."

Apologies to all if I caused irritation. I shall try to avoid the word "plinker" in future!