Whobilly: " Will a worn out Breech cause inconsistencies in groups? "
It's the flash hole that erodes.... as they open up they tend to let more back pressure to the primer... it seems that once they open up how much the change in pressure varies from shot to shot... this tends to open up the groups. You will find lost of folks with different ideas on how big a flash hole should be and when it is worn out. Our vent liners are a type of replaceable flash hole. They start with a hole that is about .030 and once it grows to .036 its recommended to replace it.
There are two basic designs to breech plugs
1 :with replaceable flash holes
2: Non-replaceable flash holes... ie your fix is replacing the entire breech plug or retrofitting a replacable flash hole into it
The later Knight breech plugs have vent liners.. but knight had them silver soldered or epoxied in. My guess is that their legal saw them as a liability (that a shooter could forget to put it in and blow up a primer due to excessive back pressure). Some folks (like Luke at arrowhead that was suggested above) make breech plugs with carbide bushings with holes through them. Carbide is very hard and resists the erosion. But is very expensive, along with EDM'ing holes through cabide.
We sell Lehigh's 17-4 stainless steel vent liners that have been heat treated. They work in many of the existing breech plugs that use the Savage ML-2 style vent liner, and far outlast the original Savage style (that is made out of a softer non heat treated steel alloy... black oxide finish with flat head). Some folks used to do a DIY heat treat on the original savage vent liners... but ours still outclass that by far. To me the cost of the carbide bushing makes sense in a competition rifle, where you are going to shoot and shoot, and want the peace of mind that the flash hole won't errode and cause a flier... and don't want to have to maintain your flash hole as often. But, if you are just hunting, or don't mind changing to a new vent liner before a match, vent liners make more sense to me. I believe you can buy more than 10 of them with a .036 gage pin from me for less than the cost of a bushing. A vent liner's life depends on many variables... but you can get 100 or more shots out of the stainless ones before accuracy suffers. If you have a BP that has a worn flash hole, we can retrofit a vent liner into most of them for about the cost of a new BP. Then after that, you only need to buy vent liners. This is extremely helpful with old ML's that you can't buy a new breech plug for.
If you already have a breech plug with a Savage 2 style ventiner, its a no brainer to me: buy our stainless vent liners and a gage pin (to know when to replace the vent liner before accuracy gets too bad).
This is all just my opinion. And I am here to help you and sell you a product, so you can take it for what it is worth. I hope I answered your question.