Short answer: The USGI fiberglass stocks were as accurate or better as the USGI wood stocks.
Long answer: From the 02/23/08 version of the Third Edition of M14 Rifle History and Development aka "the canon":
"USGI M14 Synthetic Stocks
The purpose for development of a synthetic material stock for the M14 rifle was two-fold: 1) to avoid a single supply source or single type of material for the stock 2) to avoid the varying quantity and quality of walnut wood. As a result of testing performed in 1961, the U. S. Army Infantry Board expected that synthetic material stocks would last twice as long as those made from walnut and that pricing for the synthetic stock would compete with walnut stocks.
Experimental and production versions of the synthetic stock were fiberglass reinforced polyester plastic shell halves glued together with polyurethane foam filling in between. Experimentation with synthetic material stocks began in the late 1950s for the M14 project as at least two T44E4 rifles were fitted with black color man-made material gunstocks. James S. Lunn and others patented a reinforced fiberglass M1 Garand rifle stock in 1959. Lunn Laminates (NY) made a number of M1 reinforced fiberglass stocks. Synthetic material stocks for the M1 and M14 rifles were evaluated by the U. S. Army Infantry Board during the first half of 1960. Due to similarity with the Lunn design M1 stocks, the firm may have been involved in the 1960 development for the M14 reinforced fiberglass stock but this has not been confirmed.
General Tire & Rubber Company joined Springfield Armory in 1960 in conducting research to develop a synthetic stock for the M14 rifle. Development of this lighter, stronger stock made of fiberglass reinforced polyester plastic for the M14 proceeded in stages from 1960 until late 1965. The drawing for the final version M14 fiberglass stock is dated December 1965. Beginning in 1962, fiberglass stocks were installed on M14 rifles at the factory but it was not by Springfield Armory. The following year, Springfield Armory produced a single batch of 10,000 M14 rifles in October 1963 with synthetic material stocks.
The early synthetic material stocks had no checkering and the hole for the upper butt plate screw had a wood insert to which the wood stock upper butt plate screw was anchored. These early issue synthetic stocks were milk chocolate brown in color. At least some of these early stocks were marked on the inside of the magazine well as follows: right side top line - DM-1775-GB-2, right side bottom line - DT, left side top line - DT and left side bottom line - DM-1775-GB-1. Later-manufacture fiberglass stocks were assembled with an upper butt plate screw with a finer thread. The later upper butt plate screw is anchored to a steel nut held inside a metal bracket inside the stock. 1961 pre-production synthetic stocks were found satisfactory for mounting the M15 grenade launcher sight. If a M15 grenade launcher sight was mounted on a 1961 synthetic M14 stock, the screws had to be mounted about 1/8 " lower than what was done on the wood stock. This was to avoid protrusion of the top screw into the magazine well creating interference with fitting the receiver. This fitment issue was solved by the time the 1965 design was drawn up.
Stock ferrules on synthetic USGI stocks are either crimped or dimpled but the dimpled ferrules are not common. Military Specification MIL-S-45921A and drawing F11686427 require the ferrule to be cemented and crimped to the stock to prevent movement. Thus, it appears that dimpled ferrules are earlier production stocks.
The USGI M14 synthetic stock was designed and tested for ruggedness. Forty sample stocks were selected by the U. S. government representative during initial production and monthly thereafter. Any failure of any test of any sample stock meant the day’s production or the represented batch was rejected.
Ten tests were conducted on each sample lot of production fiberglass reinforced plastic stocks: 1) physical examination 2) functioning firing 3) targeting and accuracy 4) adhesive and foam 5) twist 6) shock resistance 7) shock resistance endurance 8) low temperature shock resistance 9) low temperature abuse and 10) heat resistance. Some tests were conducted on all forty sample stocks, e.g., visual, functioning firing and targeting and accuracy. Most of the tests though were conducted on a portion of the sample lot. For example, twist testing was performed on five stocks and shock resistance was tested on ten stocks in each sample lot. The sample stock testing is briefly reviewed here:
Physical Examination – The sample stock was examined for proper assembly and measured for compliance to specified dimensions at several points. Both sides of the synthetic stock were tested twice each at the butt, the receiver and the forearm areas for hardness for a total of twelve readings per stock. The readings were required to average a minimum of 50 Barcol. The Barcol scale is used to measure the hardness of reinforced and non-reinforced rigid plastics.
Functioning Firing – The stock was assembled with other components to make a complete M14 rifle. The rifle was placed into a fixture to simulate offhand firing. The rifle was test fired using two full twenty round magazines in both semi-automatic and automatic using M59 or M80 ball ammunition.
Targeting and Accuracy – Again, the sample stock was made part of a complete M14 rifle and placed into a test fixture. The rear sight aperture was set at eight clicks from bottom and zero windage. The rifle sights were aligned at the 6 o’clock position on a target and fired for ten rounds. The results of the sample lot of forty synthetic stocks were compared to the targeting and accuracy results of forty wood stocks known to be satisfactory. The targeting and accuracy results of the synthetic stock sample lot had to equal or exceed those of the forty wood stocks to pass.
Adhesive and Foam – The stock was sectioned by cutting at a minimum of eleven specified areas along the length of the stock. The foam was examined for any presence of voids. The specific requirement was that the foam fill had to be free of voids more than 10 % of the total volume with none more than ¼” in diameter and no more than one void per two cross-sectioned areas.
Twist – The sample stock was mounted into a test fixture and torqued to 40 ft-lbf once each in the clockwise and counterclockwise directions. The stock was visually examined for any defects.
Shock Resistance – The stock was assembled with other components to make a complete M14 rifle. The M76 grenade launcher was attached to the flash suppressor and the spindle valve turned to the horizontal (gas cutoff) position. The rifle was placed into a fixture. With the toe of the butt stock at 45 degrees, five practice anti-tank grenades were launched using M64 grenade blank cartridges. The sample stock was then removed from the rifle and examined for any defects or signs of damage.
Shock Resistance Endurance – Three of forty stocks in each sample lot were selected for this test. It is performed in the same manner as the shock resistance test but 100 rifle grenades are fired instead of five using each sample stock.
Low Temperature Shock Resistance Test – This test was performed just like the shock resistance test except the rifle was kept at – 65 degrees Fahrenheit + or – 5 degrees for three hours before and then during the test.
Low Temperature Abuse Test – The sample stock assembled into a complete M14 rifle was stored at – 65 degrees Fahrenheit + or – 5 degrees for three hours then dropped from a height of 3 feet on to a hardwood bench. The test was repeated so that the stock would impact at different points (left side, right side, and butt stock toe and heel) on the bench. After this, the rifle was allowed to swing 90 degrees by its own weight from a horizontal position, while held at the muzzle, so that the stock would hit the flat side of the hardwood bench. This was repeated so top, bottom and both sides of the stock impacted the side of the bench.
Heat Resistance – The sample stock was assembled into a M14 rifle with the bolt, firing mechanism and operating rod spring removed. The test rifle was placed in a stand to hold it in the horizontal position. A 115 Volt AC, 1000 Watt heating rod with an effective heating length of 24 " was placed inside the barrel. The heating rod was connected to a variable voltage control. A thermocouple was attached to the top of the barrel about 10.5 " from the end of the muzzle. The thermocouple was wired to a pyrometer to indicate the barrel temperature during the test. By increasing the voltage applied to the heating rod in stages, the rifle barrel temperature was brought up to 1,200 degrees Fahrenheit. At that point, electric current flow through the heating rod was stopped and the barrel cooled to ambient temperature. The stock was then examined for any damage.
USGI M14 synthetic stocks have the letters DT as well as other alphanumeric characters in the magazine well area. A small gap between the middle portion of the receiver and the stock is normal for USGI wood and synthetic M14 stocks, but is more noticeable on many fiberglass stocks. The USGI synthetic M14 stock itself weighs 34 to 36 ounces. A USGI M14 synthetic stock with all the correct metal hardware weighs in at about 46 ounces. The USGI M14 synthetic stock will become too hot to hold if left outdoors for as little an hour in the direct sun. If the synthetic stock is brought into the shade for two minutes or more, it will cool sufficiently to be comfortable to handle. The USGI M14 synthetic stock will burn after 2 minutes 10 seconds of exposure to an open flame. Both wood and synthetic M14 stocks are strong enough to break the operator’s fall to a prone shooting position from a run. Each type of stock will withstand fifty vertical butt strokes against a test dummy but will suffer damage in less than fifty horizontal butt strokes against the same butt stroke dummy. The damage is likely to occur in the firing hand grip area of the stock. USGI M14 synthetic stocks have always been allowed for competition at Camp Perry.
USGI synthetic M14 stocks were made as late as 1968. The U. S. Army awarded a contract to General Tire & Rubber Company in 1968 for 500,000 synthetic M14 stocks to be used as replacement stocks. At about $2,187,000.00, the February 1968 contract was the single largest known dollar amount awarded for production of a M14 rifle part ever. According to an employee working at the lab during the period, a product improvement program had been written up at the General Thomas J Rodman Laboratory at Rock Island Arsenal in the early 1970s. In 1972, the Army reorganized the arsenal system. As part of the reorganization, the Rodman Laboratory staff was reduced from 125 employees to a skeleton crew of twenty-one. The M14 stock improvement program never went any further."