[constructor]

Some reloaders check to find the lands so they can back off .020 or whatever they think is the best distance off the lands. Many use the Hornady lock and load OAL gauge similar to this red one below. These work fine just check 3-4 times to get a good reading. Other people use a piece of sized brass with a slot cut in the end. This is where things get a little more complicated. You must use new or sized brass so it will fit loosely in the chamber but you must expand the neck just enough to let the bullet slide in with some tension and the ball expander in the die set may not be big enough. I know a lot of people will say that is why the slot is there but if you depend on the slot only the 2 cut edges of the neck will touch the bullet and that will force the neck apart making it wide perpendicular to the slot. Sometimes that is enough to cause an interference fit in the chamber neck and cause the bullets to jam the lands hard and possibly give a false reading. Also after slicing the neck you must make sure there are no burrs where it was cut or at the case mouth. I try to do this before I mount the barrel in an upper, it is easier to get to especially if you have to use needle nose pliers to remove the case from the chamber. Below is a case sliced into 4 parts, it may be better just make sure there are no burrs, it only takes 1 burr .001-.002" thick to make a difference.
ETA -Most 6.8 bullets can be set to 2.295" maximum a normal mag will allow and still not touch the lands.

 

[greentick]

You also have to beware of the projo "sticking" as you withdraw, giving you a falsely long reading.

I only really had an issue with 90GDs. I just kept fiddling with the coal until it didn't stick.

 

[RepOfTx]

My experience with AR platform rifles are that the throats are very long and loading rounds .020 off the lands would prevent the rounds being loaded into an magazine. I load for F-Open competition with a long range bolt action rifle and the chambers cut to your specifications. There are competitors who shoot open site service rifles at 600 and 1000 yards and those rounds are loaded one at a time for each shot. The .223 people are shooting 79 grain match ammo with custom AR barrels and chambers.

 

[BillB]

I use the LNL from hornady, straight model, and do as the OP said and do it before the narrel is installed. Ill take 5 readings with any bullet I think I might shoot in that barrel and record each individual measurement and average them out and record all info in my data log.

I didnt anticipate ever shooting any 130gr fusions in my 6.8's but found 500 of them for a song and with the straight model. No, I dont even chase the lands but with those 130s its nice to know where Im at in there.

 

[constructor]

My experience with AR platform rifles are that the throats are very long and loading rounds .020 off the lands would prevent the rounds being loaded into an magazine. I load for F-Open competition with a long range bolt action rifle and the chambers cut to your specifications. There are competitors who shoot open site service rifles at 600 and 1000 yards and those rounds are loaded one at a time for each shot. The .223 people are shooting 79 grain match ammo with custom AR barrels and chambers.

If all you shoot is a 5.56 then yeah most but, how long can you load the 120smk in a 6.5 grendel before you hit the lands, 115gr Hornady hpbt in a 6.8, 90gr Lapua in a TAC6? A lot of the guys here shoot more than just a 5.56 and that is why we have topics like this.