he!!o, seeking advise how to  you  bench   to  zero in your rifle , I can bench shoot at 100, 200 & 400 yards,

do you use a sand bags or a machine , please advise, i am a pistol shooter seeking a new avenue

I use an adjustable rest with its own sandbag & a "Bunny Ears" rear bag I got with it. You do NOT need the super duper super pricey high end units, anything well made is all.

Caldwel make some nice ones.

https://www.caldwellshooting.com/rests/

You can use just a sandbag on a suitable riser, like an ammo can & another under the stock though. 

You do NOT hold the rifle "Normally".

The front bag is under the front wood about where your hand would be when shooting normally.

The trigger hand holds the stock wrist & the pad of the trigger finger rests on the trigger when relaxed.

The off hand does not hold the rifle at all. It bends back at the elbow so the crotch of the thumb & first finger fits under the bottom of the butt stock, & the spread fingers grasp the right shoulder.

If you use a rear bunny ears you grasp the bag & squeeze it for final sighting adjustments, rough adjustments are made with the screw elevator for the front rest.

The sling is not used.

I'd rough zero @ 25 yds, If the rifle is close but no cigar start at 100 yds.

You will notice that you don’t sit behind the rifle, you sit on the side of the bench, body faces 90 degrees to the line of fire. 

Your left hand (for a right handed shooter) can fine tune the aim by squeezing the sandbag under the butt of the rifle.  Once I have good sight alignment, I hold the back of the butt stock between my fingers and thumb of my left hand, just in front of and below the shoulder. 

Once I have established the rifle/handload combination that provides the most accuracy then I start to shoot the rifle off sandbags, or even unsupported if a match demands it.  Remember that many rifles can have a slight change in POI in the transition from Lead Sled to being held when shooting.

I tried the LED SLED & didn't like it. I went back to the setup shown above. before deciding on your bench kit try them out as its an expensive mistake to pick the wrong type!

I always set up on the bench such that the fore-end is sitting in the front rest about at the same position the support hand is while shooting prone.  There may be some change in POI between bench rest and prone, but doing it this way it is minimized. You don’t want a fully rigid support. 

Yes this is my thinking also.

I made a front bag support out of a cooler back for a six pack of beer. With three zip lock plastic bags of sand. One in the bas and two in the top, to allow a slight hollow channel for the rifle. I only support the front with a bag, using my left hand to support the butt in my shoulder. It's good enough for testing ammunition loads, it's not a bench rest rifle.

Once you've finished your load development and worked up an accurate handload, now you set up your rifle up at the bench to sight it in.  This is when I shoot off a tripod and sandbags and ensure the POA and POI coincide as well as build a range table so I know which setting on the aperture sight matches the ballistics of my load as well as what shooting with my input (fatigue, flinch, shakes, poor eyesight; whatever) does to my accuracy as you will find when shooting from a bench or prone at a meet.

You may find other ways to do the same thing but with the cost of ammo whether you buy commercial or roll your own investing in a Lead Sled soon pays for itself, again in my opinion.  Others may hold contrary positions.

Yes indeed!

Over my years of shooting various forms of “Service Rifles”, my thoughts on load development have evolved somewhat.  If it’s a new shooter who may be prone to flinching, the more stable the rifle is held, the better. 

My usual procedure for load testing was to fire at least four 10 shot groups off the bench of each load, 40 rounds.  This explodes very quickly if you want to test different powders, different charge weights, different primers, different bullets, different cartridge overall lengths. Heck, you can shoot a barrel out just doing load development this way!  

The problem when it comes to shooting these old military surplus rifles is we are also dealing with things like bore condition, stock bedding, and a host of other things that can affect accuracy. We shoot one 5 shot group and it’s 1 MOA, great, we call it done! The next 5 shot group might be 4 MOA, what happened??  Is it the rifle, the load, the shooter?   

If we are dealing with say a new AR-15 for Service Rifle match shooting, we can probably eliminate the rifle as a cause of poor shooting.  With a good AR, just about any safe load should group quite well with an appropriate bullet.   

I’m a bit embarrassed to say I shot thousands of rounds, tried 3 different barrels, experimented with different loads to get my T to shoot as it should.  No amount of “load development” got me the results I wanted.  I may not be out of the woods entirely, but I see some big improvements now that specific rifle and scope issues were remedied. To this, I’ve learned that you should first give an honest evaluation on how well the rifle shoots with ammunition intended for that rifle, or with a load that reasonably replicates the service ammunition.  If groups are wild, you probably have a rifle issue. 

I’m now a fan of the “ladder test” method.  It is a very efficient load development approach. We look for a load that is fairly insensitive to bullet muzzle velocity variations by plotting the mean POI on the target of each load in 0.2 to 0.3 gr increments.   I do this with 5 shots of each powder charge, but can be done with just 3 shots.  

This is all probably way too much information for someone who just wants to sight the rifle in!   And what Zed suggested is entirely suitable for that purpose.  

i use shammu/brittsrifles method for sight in with a target thats gridded in inches 

I would recommend the PPU .303 174 grain bullet is a good place to start. Keep the empty brass for reloading later on.

It will show you how good the rifle is and how good you are. Later on you can work on reloading to get the extra bit of accuracy and regularity.