My McHale LBP 36 with everything to include the by-pass harness, P&G Extensions, and even the bear canister straps under the lid weighs 4 lbs,9.5 ounces. This is at 79 liters capacity.
For most trips it is under 4 lbs. If I removed all the straps and extra webbing I could probably get it down to under 3 lbs. I have gotten it down to 3 lbs, 6 oz on a couple trips. Since I bought the smaller McHale Bump for these kinds of trips, there is no need to try and trim down the LBP.