Zachary,
Actually the inserts are not that heavy. For one thing you have to remove some of the wood from the butt stock. For another I find most rifles .375 H&H and larger to be barrel heavy. In the Ruger and Winchester rifles I have them in they actually helped improve the balance of the rifle. I use the C&H Research brand of inserts and the following is a take on that brand with a listing of their models and weights. As far as mounting one in a plastic stock I have no idea as I don't own a rifle with a plastic stock.
C&H RESEARCH
MERCURY RECOIL SUPPRESSOR
Uses Inertia To Absorb Painful Recoil
Helps counteract the painful effects of recoil using mercury, one of the heaviest, densest, liquid elements. Compact, permanently-sealed, steel case is made from bar stock; the only moving part is the liquid mercury so there's no wear, no chance of leaks. Mercury's viscosity varies only 1% from 0° F. to 90° F. which means the recoil-fighting performance is consistent in almost all weather conditions. Stock Models fit in rifle or shotgun stocks and are drilled and tapped ¼"-28 tpi for easy installation and removal. Clamp-On models include a sturdy, black anodized, non-marking, aluminum clamp for instant installation onto barrels .780" to .950" in diameter. Chamber model is attached to a 12 gauge chamber snap cap and drops into the unused barrel of an O/U when shooting singles. Magazine Tube fits into the 4-round magazine tube of a pump or semi-auto shotgun. Replaces the migratory bird plug. Includes nylon spring seat adapters for 12 gauge guns, also fits 20 gauge shotguns with adapters removed. Benelli Super - Replaces magazine cap. Benelli Montefeltro - Threads onto the bolt extending from front of magazine. Beretta 303 - Replaces the magazine cap on AL-2, 301, 302 and 303 Beretta semi-automatic shotguns. Beretta 390 and 391 - Replaces the magazine cap.
T-Handle - Makes it really easy to install and remove all stock models.
SPECS: Machined, stainless steel tubes.
Short Stock - 3" - 3" (7.6cm) x .875" (22mm) dia., 8 oz. (227 g) wt.
Standard Stock - 4" - 4" (10cm) x .875" dia., 11 oz. (312 g) wt.
Standard Stock - 5" - 5" (12.7cm) x .750" (19mm) dia., 11 oz. wt. **
Clamp-On 6 oz. - 2" (5cm) long x .750" dia., 6 oz. (170 g) wt.
Clamp-On 8 oz. - 3" long x .750" dia., 8 oz. (227 g) wt.
Chamber - 5" long x .750" dia., 11 oz. wt. Magazine Tube - 5" (12.7cm) long x .750" (19mm) dia., 11 oz. wt.
Benelli Super - Flat black finish, fits Super Black Eagle and M1 Super 90, 4½" (11.4cm) long, 8½" oz. (241 g) wt.
Benelli Montefeltro - Flat black finish, fits Montefeltro, Legacy, Sport, Competition. 4" (10cm) long, 8 oz. (227 g) wt.
Beretta 303 - 3¼" (8.2cm) long x .760" (2cm), 7 oz. (198 g) wt.
Beretta 390 - 3.6" (9.1cm) long x .49" (1.24cm). 8.2 oz. (232 g) wt.
Beretta 391 - 4" (10.1cm) long x .750" (19mm) dia. 8.4 oz. (234 g).
T-Handle - Aluminum handle, stainless steel shaft. ¼" -28 threads, 3.25" (8.2cm) x 6" (15.2cm).**
Stock No. Product Price
163-100-004 Short Stock Recoil Suppressor $47.50
163-100-002 Standard Stock 4" Recoil Suppressor $47.50**
163-000-001 Benelli Super Recoil Suppressor $65.00
163-000-002 Benelli Montefeltro Recoil Suppressor $65.00
163-101-006 Clamp-On 6 oz. Recoil Suppressor $55.00
163-101-008 Clamp-On 8 oz. Recoil Suppressor $55.00
163-100-003 Chamber Recoil Suppressor $47.50
163-100-001 Magazine Tube 5" Recoil Supressor $47.50
163-100-006 Beretta 303 Recoil Suppressor $55.00
163-100-007 Beretta 390 Recoil Suppressor $55.00
163-000-003 Beretta 391 Recoil Suppressor $55.00
163-100-000 T Handle $7.50**
** = Denotes Purchased Model