“Parallel, square, straight, concentric and true, extreme rifle accuracy lives and dies by you.”
As you may recall, we began our January 2012 “Basic Blueprinting” article with the exact same first sentence now appearing here. Considering the critical relationship between receiver face, washer style recoil lug and precisely squared barrel shoulder, it seemed only “fitting” to continue with this same truism.
In our receiver blueprinting article we went to great lengths “setting up” and “indicating in” the receiver to insure that it was spinning on its true center axis in the lathe. Once this was accomplished, we made multiple light cuts across the receiver face until we reached “full clean-up” and theoretically produced a receiver face that is perfectly square to the center axis of the body. I say “theoretically” only because experience has shown that normal humans, in normal shop environments, using normal measuring tools are normally only capable of about plus or minus 0.0002”- 0.00025” accuracy. Therefore, to state that it is “perfectlysquare” is probably an incorrect statement; then, again, we have always considered being “normal” as highly overrated – so who knows, maybe some of them really are “perfectly” square. Just remember that extraordinary standards are essential to producing extraordinary results; while absolute machining perfection may or may not actually be attainable, if you strive for flawless perfection in every detail throughout the entire build process, the resulting groups will surely reflect that effort.
Now, I will assume that most of our readers are familiar with the standard Remington model 700 receiver. I will also assume that most of you are equally aware of the many custom shop “700 clones” that are produced to duplicate (with minor alterations) this type of receiver.
I will further assume that a majority of our readers realize that all Remington 700’s and most of the 700 clones use a “captured washer” type recoil lug which is tightly sandwiched between the barrel shoulder and the receiver face. (See photo #1)
But, what I bet most laypersons do not know is just how pivotal a role that “washer-lug” plays in the accuracy potential of the finished rifle. Think of it this way, all the work required in producing a barrel shoulder perfectly square to the bore and a receiver face perfectly square to the body, is completely wasted unless both sides of the recoil lug are perfectly parallel, uniform in thickness and dead nuts flat. After all, for better or worse, that lug is going between those other two theoretically perfect surfaces and any imperfection in the lug will be transferred to that otherwise flawless joint. But, here is the real kicker; in my experience none of the lugs are ever perfect – certainly not as they come from the big factories; and more distressingly, not as they come from any of the aftermarket producers; nope, none, never.
Now before you jump up and begin constructing your letter bomb, please let me qualify that last statement with these two important disclaimers: First, there may be someone producing “perfect” aftermarket lugs of which I am unaware; and second, of the many makes and models of lugs we have used, some of them are very, very, good indeed. (Very good for mass production.) Still, in our quest for absolute perfection, it has become our standard shop practice to individually surface grind any and all lugs regardless of make or model – and in a moment I think you will see why.
At the time of this writing, we have in our shop 34 new untouched Rem. 700 lugs taken from various parts donor rifles. (See Photo # 2)
We also have 10 new aftermarket lugs from one of the “very, very good” manufacturers; these lugs were purchased over a 4 month period from two different large distributors. These are the exact same lugs that we use in most of our 700 type builds and we purposely bought these lugs at varying times and places to avoid “one lot syndrome” hopefully giving us a more representative sample for the measurements we are about to take. (See photo #3)
Now although it should be unnecessary, I will mention that for the level of measuring accuracy required here, any caliper or cheap micrometer is unacceptable. When we measure lugs, we use our best digital Mitutoyo Quantumike that reads to 0.00005”. This is not to suggest that we (or anyone short of Superman) can actually read to 0.00005”, but every little bit helps and all those extra zeros make you look really smart.
So here is the basic protocol: on the lug surfaces that actually touch the barrel shoulder and receiver face, take precise measurements at the 12:00, 3:00, 6:00 and 9:00 O’clock positions. We normally take multiple measurements at numerous points across the lug body also, but these measurements are not as significant assuming that the barrel-lug-receiver assembly is properly and professionally bedded. (The front, sides and bottom of a recoil lug should never touch anything inside the stock; only the back of the lug should touch the bedding.)
The results for the 34 factory lugs are as follows:
Truthfully, I am somewhat surprised with the data for these factory lugs. Although we have measured many, many lugs individually, this is the first time we have ever measured and recorded thirty four at once and the results are much more uniform than I would have anticipated. Excluding the fifth digit to the right of the decimal point for simplicity, we see a minimum deviation of 0.0003” in lug #24; we also see lug #11 and #31 with a deviation of only 0.0004”, and lug #17 and #18 both with a deviation of 0.0005”. Therefore, out of thirty four factory lugs, five had deviations of 0.0005” or less; which is totally unexpected and pretty darn impressive.
The two worst lugs were #7 and # 15 both with deviations of 0.0023” - which is like Mount Everest in the precision gun building world. In a moment we will place lug # 7 on the surface grinder and see if we can turn a sow’s ear into a silk purse.
But first, the results for the ten “aftermarket” lugs.
The data for the 10 aftermarket lugs are as follows:
Holy cow, another ten or twenty lugs and I might have blown a head gasket! Whoever said writing isn’t hard work obviously never did much of it.
Once again, disregarding the fifth digit to the right, we note that only lug #2 and #3
exceeded 0.0005” deviation, and then only by 0.0001”.
(To keep things in perspective, the page this is written on measures a whopping 0.0022”.)
Lugs #1, #5, #6, and #8 all had a deviation of 0.0003” or less with the other four lugs falling between those highs and lows - definitely silk purse territory - but still not absolute perfection.
Now, without going broke or ending up in a rubber room, let’s fire up the grinder and see if we can improve upon these lug dimensions. (See photo #4)
For our little skill test/experiment, we will use factory lug #7 and aftermarket lug #2; the two worst lugs from each group. We will paint both lugs with Dykem layout fluid and attempt to photograph the progressive “clean-up” of the high spots on each lug. (A lot easier said than done.)
Photos #5, 6, 7 and 8 show factory lug #7 in various stages of clean-up on side number one. Photos # 9 and 10 show the clean-up of the opposite side.
Photo #11 shows the clean-up of side one on the aftermarket lug; photo #12 shows the near completion of the other side. If you note the “clean-up patterns” in the photos, you will see that they closely align with the deviation measurements from the charts - which is to be expected.
So, did we actually gain anything from all this work? Does a rich guy in a chauffeur driven Rolls really eat Gray Poupon - but of course!
The original measurements for factory lug # 7 were:
So, is it possible, with a lot of additional time and work, to get even closer to absolute perfection? Well, if you are willing to get really wacky you could turn the lugs around and grind both sides again in the opposite direction of the first pass; and then, you could go over them once more cross grain to the other passes!
But be warned, that level of extremism could lead to bunking with Sarah Conner at Pescadero. Or, one hole rifles.